chips are commonly found, it makes sense to have it in GENERIC. This
is a candidate for a RELENG_6 MFC.
Approved by; peter
Requested by: pav
Tested by: pav
and return a printable representation.
This fixes recognition of the PC Engines WRAP and improves the
recognition of the Soekris boards (Bios version can now be
seen in the dmesg output for instance).
Also, add watchdog support for PCM-582x platforms.
Submitted by: Adrian Steinmann <ast@marabu.ch>
Slightly changed by: phk
PR: 81360
by Vladimir Dergachev for inclusion in DRM CVS, with minor modifications for
FreeBSD CVS and the appropriate license from Nicolai Haehnle on r300_reg.h.
Fixes hangs when using r300.sf.net userland, tested on a Radeon 9600 on amd64.
variables rather than void * variables. This makes it easier and simpler
to get asm constraints and volatile keywords correct.
MFC after: 3 days
Tested on: i386, alpha, sparc64
Compiled on: ia64, powerpc, amd64
Kernel toolchain busted on: arm
- Make sure timer0_max_count is set to a correct value in the lapic case.
- Revert i8254_restore() to explicitly reprogram timer 0 rather than
calling set_timer_freq() to do it. set_timer_freq() only reprograms
the counter if the max count changes which it never does on resume. This
unbreaks suspend/resume for several people.
Tested by: marks, others
Reviewed by: bde
MFC after: 3 days
address, writting non-canonical address can cause kernel a panic,
by restricting base values to 0..VM_MAXUSER_ADDRESS, ensuring
only canonical values get written to the registers.
Reviewed by: peter, Josepha Koshy < joseph.koshy at gmail dot com >
Approved by: re (scottl)
further changes and fixes in the future:
- Use aliases via macros rather than duplicated inlines wherever possible.
- Move all the aliases to the bottom of these files and the inline
functions to the top.
- Add various comments.
- On alpha, drop atomic_{load_acq,store_rel}_{8,char,16,short}().
- On i386 and amd64, don't duplicate the extern declarations for functions
in the two non-inline cases (KLD_MODULE and compiler doesn't do inlines),
instead, consolidate those two cases.
- Some whitespace fixes.
Approved by: re (scottl)
copied and pasted. I had actually tested without this change in my
trees as had the other testers.
Reported by: bde, Rostislav Krasny rosti dot bsd at gmail dot com
Approved by: re (scottl)
Pointy hat to: jhb
packet filter. This would cause a panic on architectures that require strict
alignment such as sparc64 (tier1) and ia64/ppc (tier2).
This adds two new macros that check the alignment, these are compile time
dependent on __NO_STRICT_ALIGNMENT which is set for i386 and amd64 where
alignment isn't need so the cost is avoided.
IP_HDR_ALIGNED_P()
IP6_HDR_ALIGNED_P()
Move bridge_ip_checkbasic()/bridge_ip6_checkbasic() up so that the alignment
is checked for ipfw and dummynet too.
PR: ia64/81284
Obtained from: NetBSD
Approved by: re (dwhite), mlaier (mentor)
by amd64 and i386: For buffered writes we collect data and write it
out a ${DEV_BSIZE}-sized block at a time. The fragsz variable is used
to keep track of how much data we have collected in the buffer so far
and it's reset to zero immediately after writing a block to the dump
device.
When the last, possibly partially filled buffer is flushed, we didn't
reset fragsz to 0 and as such would stop reflecting reality. Since we
currently only need to do buffered writes once, this isn't a problem.
However, when kernel dumps are made by hand (say by callling doadump
from within DDB), the improperly cleared state from the first call to
dumpsys causes the next call to dumpsys to create an invalid code file.
This change resets fragsz after flushing the partially filled buffer so
that it fixes the two problems at once.
Approved by: re (scottl)
This is good enough to be able to run a RELENG_4 gdb binary against
a RELENG_4 application, along with various other tools (eg: 4.x gcore).
We use this at work.
ia32_reg.[ch]: handle the 32 bit register file format, used by ptrace,
procfs and core dumps.
procfs_*regs.c: vary the format of proc/XXX/*regs depending on the client
and target application.
procfs_map.c: Don't print a 64 bit value to 32 bit consumers, or their
sscanf fails. They expect an unsigned long.
imgact_elf.c: produce a valid 32 bit coredump for 32 bit apps.
sys_process.c: handle 32 bit consumers debugging 32 bit targets. Note
that 64 bit consumers can still debug 32 bit targets.
IA64 has got stubs for ia32_reg.c.
Known limitations: a 5.x/6.x gdb uses get/setcontext(), which isn't
implemented in the 32/64 wrapper yet. We also make a tiny patch to
gdb pacify it over conflicting formats of ld-elf.so.1.
Approved by: re
* Add ichwd (The Intel EM64T folks have an ICH)
* Cosmetic comment syncs
* Merge cpufreq change over to NOTES
* add pbio (it compiles, but isn't useful since no boxes have ISA slots)
* copy ath settings (note: wlan disabled here since its in global NOTES)
* copy profiling, including fixing a previous i386->amd64 merge typo.
Approved by: re (blanket i386 <-> amd64 sync/convergence)
be possible to get the swapgs state reversed if doreti traps during
the iretq. Attempt to handle this. load_gs() might need special
handling too. Running the kernel with the user's TLS and the
kernel's PCPU space interchanged would be bad(TM).
Discovered as a result of a conversation with: bde
Approved by: re
dump format. The key reason to do this is so that we can dump sparse
address space. For example, we need to be able to skip the PCI hole
just below the 4GB boundary. Trying to destructively dump MMIO device
registers is Really Bad(TM). The frequent result of trying to do a
crash dump on a machine with 4GB or more ram was ugly (lockup or reboot).
This code has been taken directly from the IA64 dump_machdep.c code,
with just a few (mostly minor) mods.
Introduce a dump_avail[] array in the machdep.c code so that we have a
source of truth for what memory is present in a machine that needs to be
dumped. We can't use phys_avail[] because all sorts of things slice
memory out of it that we really need to dump. eg: the vm page array
and the dmesg buffer. dump_avail[] is pretty much an unmolested version
of phys_avail[]. It does have Maxmem correction.
Bump the i386 and amd64 dump format to version 2, but nothing actually
uses this. amd64 was actually using the i386 dump version number.
libkvm support to follow.
Approved by: re
exec_copyin_strings() to catch up to rev 1.266 of kern_exec.c. This fixes
panics on amd64 with compat binaries since exec_free_args() was freeing
more memory than these functions were allocating and the mismatch could
cause memory to be freed out from under other concurrent execs.
Approved by: re (scottl)
vm_page's machine-dependent fields. Use this function in
vm_pageq_add_new_page() so that the vm_page's machine-dependent and
machine-independent fields are initialized at the same time.
Remove code from pmap_init() for initializing the vm_page's
machine-dependent fields.
Remove stale comments from pmap_init().
Eliminate the Boolean variable pmap_initialized from the alpha, amd64,
i386, and ia64 pmap implementations. Its use is no longer required
because of the above changes and earlier changes that result in physical
memory that is being mapped at initialization time being mapped without
pv entries.
Tested by: cognet, kensmith, marcel
- Implement sampling modes and logging support in hwpmc(4).
- Separate MI and MD parts of hwpmc(4) and allow sharing of
PMC implementations across different architectures.
Add support for P4 (EMT64) style PMCs to the amd64 code.
- New pmcstat(8) options: -E (exit time counts) -W (counts
every context switch), -R (print log file).
- pmc(3) API changes, improve our ability to keep ABI compatibility
in the future. Add more 'alias' names for commonly used events.
- bug fixes & documentation.
audit event identifier associated with each system call, which will
be stored by makesyscalls.sh in the sy_auevent field of struct sysent.
For now, default the audit identifier on all system calls to AUE_NULL,
but in the near future, other BSM event identifiers will be used. The
mapping of system calls to event identifiers is many:one due to
multiple system calls that map to the same end functionality across
compatibility wrappers, ABI wrappers, etc.
Submitted by: wsalamon
Obtained from: TrustedBSD Project
address. One was alltraps_with_regs_pushed, the other was calltrap.
When the stack tracer walks up, it looks for magic symbol names to
determine how to parse non-standard stack frames, such as a trapframe.
It was looking for "calltrap". Which of the two symbols you got depended
on things like Phase of moon, etc. If you were unlucky, you got a
garbage stack trace for things like 'debug.trace_on_panic', which would
completely hide the actual source of the problem.
i8253reg.h, and add some defines to control a speaker.
- Move PPI related defines from i386/isa/spkr.c into ppireg.h and use them.
- Move IO_{PPI,TIMER} defines into ppireg.h and timerreg.h respectively.
- Use isa/isareg.h rather than <arch>/isa/isa.h.
Tested on: i386, pc98
a regular IPI vector, but this vector is blocked when interrupts are disabled.
With "options KDB_STOP_NMI" and debug.kdb.stop_cpus_with_nmi set, KDB will
send an NMI to each CPU instead. The code also has a context-stuffing
feature which helps ddb extract the state of processes running on the
stopped CPUs.
KDB_STOP_NMI is only useful with SMP and complains if SMP is not defined.
This feature only applies to i386 and amd64 at the moment, but could be
used on other architectures with the appropriate MD bits.
Submitted by: ups
in other codes. Add cpu_set_user_tls, use it to tweak user register
and setup user TLS. I ever wanted to merge it into cpu_set_kse_upcall,
but since cpu_set_kse_upcall is also used by M:N threads which may
not need this feature, so I wrote a separated cpu_set_user_tls.
inclusion of <sys/pmc.h> and depending on being included from
that header file.
o Include any MD specific header files that otherwise need to be
included from MI files.
Ok'd: jkoshy@
into _bus.h to help with name space polution from including all of bus.h.
In a few days, I'll commit changes to the MI code to take advantage of thse
sepration (after I've made sure that these changes don't break anything in
the main tree, I've tested in my trees, but you never know...).
Suggested by: bde (in 2002 or 2003 I think)
Reviewed in principle by: jhb
- Split core DRM routines back into their own module, rather than using the
nasty templated system like before.
- Development-class R300 support in radeon driver (requires userland pieces, of
course).
- Mach64 driver (haven't tested in a while -- my mach64s no longer fit in the
testbox). Covers Rage Pros, Rage Mobility P/M, Rage XL, and some others.
- i915 driver files, which just need to get drm_drv.c fixed to allow attachment
to the drmsub device. Covers i830 through i915 integrated graphics.
- savage driver files, which should require minimal changes to work. Covers the
Savage3D, Savage IX/MX, Savage 4, ProSavage.
- Support for color and texture tiling and HyperZ features of Radeon.
Thanks to: scottl (much p4 handholding)
Jung-uk Kim (helpful prodding)
PR: [1] kern/76879, [2] kern/72548
Submitted by: [1] Alex, lesha at intercaf dot ru
[2] Shaun Jurrens, shaun at shamz dot net
a return instruction. (The latter is discouraged by the Opteron
optimization manual because it disables branch prediction for the return
instruction.)
Reviewed by: bde
critical_enter() and critical_exit() are now solely a mechanism for
deferring kernel preemptions. They no longer have any affect on
interrupts. This means that standalone critical sections are now very
cheap as they are simply unlocked integer increments and decrements for the
common case.
Spin mutexes now use a separate KPI implemented in MD code: spinlock_enter()
and spinlock_exit(). This KPI is responsible for providing whatever MD
guarantees are needed to ensure that a thread holding a spin lock won't
be preempted by any other code that will try to lock the same lock. For
now all archs continue to block interrupts in a "spinlock section" as they
did formerly in all critical sections. Note that I've also taken this
opportunity to push a few things into MD code rather than MI. For example,
critical_fork_exit() no longer exists. Instead, MD code ensures that new
threads have the correct state when they are created. Also, we no longer
try to fixup the idlethreads for APs in MI code. Instead, each arch sets
the initial curthread and adjusts the state of the idle thread it borrows
in order to perform the initial context switch.
This change is largely a big NOP, but the cleaner separation it provides
will allow for more efficient alternative locking schemes in other parts
of the kernel (bare critical sections rather than per-CPU spin mutexes
for per-CPU data for example).
Reviewed by: grehan, cognet, arch@, others
Tested on: i386, alpha, sparc64, powerpc, arm, possibly more
compiler features tests. This is ok, since machine/ieeefp.h is an internal
interface. But floatingpoint.h is a public interface and some ports use it,
so include sys/cdefs.h in the amd64 and i386 version of floatingpoint.h.
Note: some architectures don't provide recursive inclusion protection in
floatingpoint.h, namely alpha and ia64. Except for this part and now the
include of sys/cdefs.h, all those files are equal (from a compiler POV),
so they could be moved to only one version in src/include/.
Approved by: joerg
There are too many questions in freebsd-amd64@ about how to enable Linux
support that it seems a required piece of functionality. Thus we should
just have it on by default.
objdump --disassemble when disassembling itself in userland. I've added
the cmovCC instruction group and tweaked a bunch of size sensitive array
indexes to either fix my mistakes and/or force it to work by any means
necessary.
I'm committing this because it is usable enough to see what is going on
when single stepping via ddb.
It might still tell lies, but its lies will be far more subtle now. I'm
not sure that this is a good thing or not.
instructions as it was when I dropped it back in May 31, 2003. I'm
committing this as an intermediate stage because back then I thought I
understood what I was doing with this file.
validation error in procfs/linprocfs that can be exploited by local
users to cause a kernel panic. All versions of FreeBSD with the patch
referenced in SA-04:17.procfs have this bug, but versions without that
patch have a more serious bug instead. This problem only affects
systems on which procfs or linprocfs is mounted.
Found by: Coverity Prevent analysis tool
Security: Local DOS
FreeBSD based on aue(4) it was picked by OpenBSD, then from OpenBSD ported
to NetBSD and finally NetBSD version merged with original one goes into
FreeBSD.
Obtained from: http://www.gank.org/freebsd/cdce/
NetBSD
OpenBSD
This is mentioned in the Handbook but it is not as obvious to new
users why bpf is needed compared to the other largely self-explanatory
items in GENERIC.
PR: conf/40855
MFC after: 1 week
where having this disabled was actually hurting us, since so many
BIOSes include legacy USB emulation that takes control of all usb
ports and only the ehci driver knows how to disable it.
to mistakes from day 1, it has always had semantics inconsistent with
SVR4 and its successors. In particular, given argument M:
- On Solaris and FreeBSD/{alpha,sparc64}, it clobbers the old flags
and *sets* the new flag word to M. (NetBSD, too?)
- On FreeBSD/{amd64,i386}, it *clears* the flags that are specified in M
and leaves the remaining flags unchanged (modulo a small bug on amd64.)
- On FreeBSD/ia64, it is not implemented.
There is no way to fix fpsetsticky() to DTRT for both old FreeBSD apps
and apps ported from other operating systems, so the best approach
seems to be to kill the function and fix any apps that break. I
couldn't find any ports that use it, and any such ports would already
be broken on FreeBSD/ia64 and Linux anyway.
By the way, the routine has always been undocumented in FreeBSD,
except for an MLINK to a manpage that doesn't describe it. This
manpage has stated since 5.3-RELEASE that the functions it describes
are deprecated, so that must mean that functions that it is *supposed*
to describe but doesn't are even *more* deprecated. ;-)
Note that fpresetsticky() has been retained on FreeBSD/i386. As far
as I can tell, no other operating systems or ports of FreeBSD
implement it, so there's nothing for it to be inconsistent with.
PR: 75862
Suggested by: bde
sys/bus_dma.h instead of being copied in every single arch. This slightly
reorders a flag that was specific to AXP and thus changes the ABI there.
The interface still relies on bus_space definitions found in <machine/bus.h>
so it cannot be included on its own yet, but that will be fixed at a later
date. Add an MD <machine/bus_dma.h> for ever arch for consistency and to
allow for future MD augmentation of the API. sparc64 makes heavy use of
this right now due to its different bus_dma implemenation.
place.
This moves the dependency on GCC's and other compiler's features into
the central sys/cdefs.h file, while the individual source files can
then refer to #ifdef __COMPILER_FEATURE_FOO where they by now used to
refer to #if __GNUC__ > 3.1415 && __BARC__ <= 42.
By now, GCC and ICC (the Intel compiler) have been actively tested on
IA32 platforms by netchild. Extension to other compilers is supposed
to be possible, of course.
Submitted by: netchild
Reviewed by: various developers on arch@, some time ago
- Add debug.watchdog tunable, so we can specify watchdog CPU from loader
which will help to debug hangs on boot.
- Remove 'U' from debug.watchdog sysctl definition, so if we set it to '-1'
it really shows '-1'.
- Fix comment.
IRQ 0 and not an ExtINT pin. The MADT enumerators ignore the PC-AT flag
and ignore overrides that map IRQ 0 to pin 2 when this quirk is present.
- Add a block comment above the quirks to document each quirk so that we
can use more verbose descriptions quirks.
MFC after: 2 weeks
pointers in argv and envv in userland and use that together with
kern_execve() and exec_free_args() to implement linux_execve() for the
amd64/linux32 ABI without using the stackgap.
- Implement linux_nanosleep() using the recently added kern_nanosleep().
- Use linux_emul_convpath() instead of linux_emul_find() in
exec_linux_imgact_try().
Tested by: cokane
Silence on: amd64
uses the i8237 without trying to emulate the PC architecture move
the register definitions for the i8237 chip into the central include
file for the chip, except for the PC98 case which is magic.
Add new isa_dmatc() function which tells us as cheaply as possible
if the terminal count has been reached for a given channel.
devclass. As pointed out by dfr@, devclasses don't have to share the same
linkage if multiple drivers have the same name. Newbus should match the
devclasses based on name and allocate non-conflicting unit numbers.
millisecond it is calibrating. Suggested by jhb@ and bde@. Don't clobber
the tsc_freq with the new value since it isn't accurate enough for
timecounters and the timecounter system as a whole needs support for
changing rates before we do this. Subtract 0.5% from our measurement
to account for overhead in DELAY. Note that this interface is for
estimating the clockrate and needs to work well at runtime so doing a full
calibration including disabling interrupts for a second is not feasible.
copies arguments into the kernel space and one that operates
completely in the kernel space;
o use kernel-only version of execve(2) to kill another stackgap in
linuxlator/i386.
Obtained from: DragonFlyBSD (partially)
MFC after: 2 weeks
cuts to the chase and fills in a provided s/g list. This is meant to optimize
out the cost of the callback since the callback doesn't serve much purpose for
mbufs since mbuf loads will never be deferred. This is just for amd64 and
i386 at the moment, other arches will be coming shortly.
from 4.x kernel config files. User's wishing to upgrade from 4.x to 6
will need to go through 5.x, or grab this script from there. These
scripts will remain in RELENG_5...
on entry and it assumes the responsibility for releasing the page queues
lock if it must sleep.
Remove a bogus comment from pmap_enter_quick().
Using the first change, modify vm_map_pmap_enter() so that the page queues
lock is acquired and released once, rather than each time that a page
is mapped.
In such cases, the busying of the page and the unlocking of the
containing object by vm_map_pmap_enter() and vm_fault_prefault() is
unnecessary overhead. To eliminate this overhead, this change
modifies pmap_enter_quick() so that it expects the object to be locked
on entry and it assumes the responsibility for busying the page and
unlocking the object if it must sleep. Note: alpha, amd64, i386 and
ia64 are the only implementations optimized by this change; arm,
powerpc, and sparc64 still conservatively busy the page and unlock the
object within every pmap_enter_quick() call.
Additionally, this change is the first case where we synchronize
access to the page's PG_BUSY flag and busy field using the containing
object's lock rather than the global page queues lock. (Modifications
to the page's PG_BUSY flag and busy field have asserted both locks for
several weeks, enabling an incremental transition.)
specified register, but a pointer to the in-memory representation of
that value. The reason for this is twofold:
1. Not all registers can be represented by a register_t. In particular
FP registers fall in that category. Passing the new register value
by reference instead of by value makes this point moot.
2. When we receive a G or P packet, both are for writing a register,
the packet will have the register value in target-byte order and
in the memory representation (modulo the fact that bytes are sent
as 2 printable hexadecimal numbers of course). We only need to
decode the packet to have a pointer to the register value.
This change fixes the bug of extracting the register value of the P
packet as a hexadecimal number instead of as a bit array. The quick
(and dirty) fix to bswap the register value in gdb_cpu_setreg() as
it has been added on i386 and amd64 can therefore be removed and has
in fact been that.
Tested on: alpha, amd64, i386, ia64, sparc64
Allocate the bounce zone at either tag creation or map creation to help
avoid null-pointer derefs later on. Track total pages per zone so that
each zone can get a minimum allocation at tag creation time instead of
being defeated by mis-behaving tags that suck up the max amount.
control the number of lines per page rather than a constant. The variable
can be examined and changed in ddb as '$lines'. Setting the variable to
0 will effectively turn off paging.
- Change db_putchar() to force out pending whitespace before outputting
newlines and carriage returns so that one can rub out content on the
current line via '\r \r' type strings.
- Change the simple pager to rub out the --More-- prompt explicitly when
the routine exits.
- Add some aliases to the simple pager to make it more compatible with
more(1): 'e' and 'j' do a single line. 'd' does half a page, and
'f' does a full page.
MFC after: 1 month
Inspired by: kris
that was greater than 4G. I originally used the same values as i386 in
order to save opening a new PML4 page slot, but in the day of gigabytes
of memory, worrying about a 4K page seems futile. Moving from 8 to 32G
moves the page to a different index, it doesn't increase the number of
pages used.
Restructure pmap_enter() to prevent the loss of a page modified (PG_M) bit
in a race between processors. (This restructuring assumes the newly atomic
pte_load_store() for correct operation.)
Reviewed by: tegge@
PR: i386/61852
the raw values including for child process statistics and only compute the
system and user timevals on demand.
- Fix the various kern_wait() syscall wrappers to only pass in a rusage
pointer if they are going to use the result.
- Add a kern_getrusage() function for the ABI syscalls to use so that they
don't have to play stackgap games to call getrusage().
- Fix the svr4_sys_times() syscall to just call calcru() to calculate the
times it needs rather than calling getrusage() twice with associated
stackgap, etc.
- Add a new rusage_ext structure to store raw time stats such as tick counts
for user, system, and interrupt time as well as a bintime of the total
runtime. A new p_rux field in struct proc replaces the same inline fields
from struct proc (i.e. p_[isu]ticks, p_[isu]u, and p_runtime). A new p_crux
field in struct proc contains the "raw" child time usage statistics.
ruadd() has been changed to handle adding the associated rusage_ext
structures as well as the values in rusage. Effectively, the values in
rusage_ext replace the ru_utime and ru_stime values in struct rusage. These
two fields in struct rusage are no longer used in the kernel.
- calcru() has been split into a static worker function calcru1() that
calculates appropriate timevals for user and system time as well as updating
the rux_[isu]u fields of a passed in rusage_ext structure. calcru() uses a
copy of the process' p_rux structure to compute the timevals after updating
the runtime appropriately if any of the threads in that process are
currently executing. It also now only locks sched_lock internally while
doing the rux_runtime fixup. calcru() now only requires the caller to
hold the proc lock and calcru1() only requires the proc lock internally.
calcru() also no longer allows callers to ask for an interrupt timeval
since none of them actually did.
- calcru() now correctly handles threads executing on other CPUs.
- A new calccru() function computes the child system and user timevals by
calling calcru1() on p_crux. Note that this means that any code that wants
child times must now call this function rather than reading from p_cru
directly. This function also requires the proc lock.
- This finishes the locking for rusage and friends so some of the Giant locks
in exit1() and kern_wait() are now gone.
- The locking in ttyinfo() has been tweaked so that a shared lock of the
proctree lock is used to protect the process group rather than the process
group lock. By holding this lock until the end of the function we now
ensure that the process/thread that we pick to dump info about will no
longer vanish while we are trying to output its info to the console.
Submitted by: bde (mostly)
MFC after: 1 month
that is no longer required. (In fact, it is not clear that it was ever
required in HEAD or RELENG_4, only RELENG_3 required a work-around.) Now,
as before revision 1.251, if the preexisting PTE is invalid, pmap_enter()
does not call pmap_invalidate_page() to update the TLB(s).
Note: Even with this change, the handling of a copy-on-write fault is
inefficient, in such cases pmap_enter() calls pmap_invalidate_page() twice.
Discussed with: bde@
PR: kern/16568
need to mask off the page offset bits. (This operation made some sense
prior to i386/i386/pmap.c revision 1.254 when we passed a physical address
rather than a vm_page pointer to pmap_enter().)
uses predate the change in the pmap_enter() interface that replaced the
page's physical address by the address of its vm_page structure. The
PHYS_TO_VM_PAGE() was being used to compute the address of the same vm_page
structure that was being passed in.
pmap_copy(). This entails additional locking in pmap_copy() and the
addition of a "flags" parameter to the page table page allocator for
specifying whether it may sleep when memory is unavailable. (Already,
pmap_copy() checks the availability of memory, aborting if it is scarce.
In theory, another CPU could, however, allocate memory between
pmap_copy()'s check and the call to the page table page allocator,
causing the current thread to release its locks and sleep. This change
makes this scenario impossible.)
Reviewed by: tegge@
