They provide relaxed-ordered atomic access semantic. Due to the
FreeBSD memory model, the operations are syntaxical wrappers around
the volatile accesses. The volatile qualifier is used to ensure that
the access not optimized out and in turn depends on the volatile
semantic as implemented by supported compilers.
The motivation for adding the operation is to help people coming from
other systems or knowing the C11/C++ standards where atomics have
special type and require use of the special access operations. It is
still the case that FreeBSD requires plain load and stores of aligned
integer types to be atomic.
Suggested by: jhb
Reviewed by: alc, jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D13534
Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.
The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.
Mainly focus on files that use BSD 3-Clause license.
The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.
Special thanks to Wind River for providing access to "The Duke of
Highlander" tool: an older (2014) run over FreeBSD tree was useful as a
starting point.
The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.
Special thanks to Wind River for providing access to "The Duke of
Highlander" tool: an older (2014) run over FreeBSD tree was useful as a
starting point.
Initially, only tag files that use BSD 4-Clause "Original" license.
RelNotes: yes
Differential Revision: https://reviews.freebsd.org/D13133
While here cache-align chains.
This shortens longest found chain during poudriere -j 80 from 32 to 16.
Pushing this higher up will probably require allocation on boot.
For processing, reclaim_pv_chunk() removes the pv_chunk from the lru
list, which makes pc_lru linkage invalid. Then the pmap lock is
released, which allows for other thread to free the last pv entry
allocated from the chunk and call free_pv_chunk(), which tries to
modify the invalid linkage.
Similarly, the chunk is inserted into the private tailq new_tail
temporary. Again, free_pv_chunk() might be run and corrupt the
linkage for the new_tail after the pmap lock is dropped.
This is a consequence of r299788 elimination of pvh_global_lock, which
allowed for reclaim to run in parallel with other pmap calls which
free pv chunks.
As a fix, do not remove the chunk from pc_lru queue, use a marker to
remember the position in the queue iteration. We can safely operate
on the chunks after the chunk's pmap is locked, we fetched the chunk
after the marker, and we checked that chunk pmap is same as we have
locked, because chunk removal from pc_lru requires both pv_chunk_mutex
and the pmap mutex owned.
Note that the fix lost an optimization which was present in the
previous algorithm. Namely, new_tail requeueing rotated the pv chunks
list so that reclaim didn't scan the same pv chunks that couldn't be
freed (because they contained a wired and/or superpage mapping) on
every invocation. An additional change is planned which would improve
this.
Reported and tested by: pho
Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
On i386, the function is used from the context switch code and needs
to be accessible externally. Amd64 MD context switch does not lock an
LDT spinlock and inlines switching in assembly.
Discussed with: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
machine independent parts of the existing code to a new file that can be
shared between amd64 and arm64.
Reviewed by: kib (previous version), imp
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D12434
The actual cache line size has always been 64 bytes.
The 128 number arose as an optimization for Core 2 era Intel processors. By
default (configurable in BIOS), these CPUs would prefetch adjacent cache
lines unintelligently. Newer CPUs prefetch more intelligently.
The latest Core 2 era CPU was introduced in September 2008 (Xeon 7400
series, "Dunnington"). If you are still using one of these CPUs, especially
in a multi-socket configuration, consider locating the "adjacent cache line
prefetch" option in BIOS and disabling it.
Reported by: mjg
Reviewed by: np
Discussed with: jhb
Sponsored by: Dell EMC Isilon
Right now, we enable the CR4.FSGSBASE bit on CPUs which support the
facility (Ivy and later), to allow usermode to read fs and gs bases
without syscalls. This bit also controls the write access to bases
from userspace, but WRFSBASE and WRGSBASE instructions currently
cannot be used, because return path from both exceptions or interrupts
overrides bases with the values from pcb.
Supporting the instructions is useful because this means that usermode
can implement green-threads completely in userspace without issuing
syscalls to change all of the machine context.
Support is implemented by saving the fs base and user gs base when
PCB_FULL_IRET flag is set. The flag is set on the context switch,
which potentially causes clobber of the bases due to activation of
another context, and when explicit modification of the user context by
a syscall or exception handler is performed. In particular, the patch
moves setting of the flag before syscalls change context.
The changes to doreti_exit and PUSH_FRAME to clear PCB_FULL_IRET on
entry from userspace can be considered a bug fixes on its own.
Reviewed by: jhb (previous version)
Tested by: pho (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks
Differential revision: https://reviews.freebsd.org/D12023
Add an option to dynamically rebalance interrupts across cores
(hw.intrbalance); off by default.
The goal is to minimize preemption. By placing interrupt sources on distinct
CPUs, ithreads get preferentially scheduled on distinct CPUs. Overall
preemption is reduced and latency is reduced. In our workflow it reduced
"fighting" between two high-frequency interrupt sources. Reduced latency
was proven by, e.g., SPEC2008.
Submitted by: jeff@ (earlier version)
Reviewed by: kib@
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D10435
Intel SGX allows to manage isolated compartments "Enclaves" in user VA
space. Enclaves memory is part of processor reserved memory (PRM) and
always encrypted. This allows to protect user application code and data
from upper privilege levels including OS kernel.
This includes SGX driver and optional linux ioctl compatibility layer.
Intel SGX SDK for FreeBSD is also available.
Note this requires support from hardware (available since late Intel
Skylake CPUs).
Many thanks to Robert Watson for support and Konstantin Belousov
for code review.
Project wiki: https://wiki.freebsd.org/Intel_SGX.
Reviewed by: kib
Relnotes: yes
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D11113
libefivar expects opening /dev/efi to indicate if the we can make efi
runtime calls. With a null routine, it was always succeeding leading
efi_variables_supported() to return the wrong value. Only succeed if
we have an efi_runtime table. Also, while I'm hear, out of an
abundance of caution, add a likely redundant check to make sure
efi_systbl is not NULL before dereferencing it. I know it can't be
NULL if efi_cfgtbl is non-NULL, but the compiler doesn't.
Some C wrappers for x86 instructions do not touch global memory and only act
on their arguments; they can be marked __pure2, aka __const__. Without this
annotation, Clang 3.9.1 is not intelligent enough on its own to grok that
these functions are __const__.
Submitted by: Anton Rang <anton.rang AT isilon.com>
Sponsored by: Dell EMC Isilon
from the top of user memory to one page lower on machines with the
Ryzen (AMD Family 17h) CPU. This pushes ps_strings and the stack
down by one page as well. On Ryzen there is some sort of interaction
between code running at the top of user memory address space and
interrupts that can cause FreeBSD to either hang or silently reset.
This sounds similar to the problem found with DragonFly BSD that
was fixed with this commit:
https://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/b48dd28447fc8ef62fbc963accd301557fd9ac20
but our signal trampoline location was already lower than the address
that DragonFly moved their signal trampoline to. It also does not
appear to be related to SMT as described here:
https://www.phoronix.com/forums/forum/hardware/processors-memory/955368-some-ryzen-linux-users-are-facing-issues-with-heavy-compilation-loads?p=955498#post955498
"Hi, Matt Dillon here. Yes, I did find what I believe to be a
hardware issue with Ryzen related to concurrent operations. In a
nutshell, for any given hyperthread pair, if one hyperthread is
in a cpu-bound loop of any kind (can be in user mode), and the
other hyperthread is returning from an interrupt via IRETQ, the
hyperthread issuing the IRETQ can stall indefinitely until the
other hyperthread with the cpu-bound loop pauses (aka HLT until
next interrupt). After this situation occurs, the system appears
to destabilize. The situation does not occur if the cpu-bound
loop is on a different core than the core doing the IRETQ. The
%rip the IRETQ returns to (e.g. userland %rip address) matters a
*LOT*. The problem occurs more often with high %rip addresses
such as near the top of the user stack, which is where DragonFly's
signal trampoline traditionally resides. So a user program taking
a signal on one thread while another thread is cpu-bound can cause
this behavior. Changing the location of the signal trampoline
makes it more difficult to reproduce the problem. I have not
been because the able to completely mitigate it. When a cpu-thread
stalls in this manner it appears to stall INSIDE the microcode
for IRETQ. It doesn't make it to the return pc, and the cpu thread
cannot take any IPIs or other hardware interrupts while in this
state."
since the system instability has been observed on FreeBSD with SMT
disabled. Interrupts to appear to play a factor since running a
signal-intensive process on the first CPU core, which handles most
of the interrupts on my machine, is far more likely to trigger the
problem than running such a process on any other core.
Also lower sv_maxuser to prevent a malicious user from using mmap()
to load and execute code in the top page of user memory that was made
available when the shared page was moved down.
Make the same changes to the 64-bit Linux emulator.
PR: 219399
Reported by: nbe@renzel.net
Reviewed by: kib
Reviewed by: dchagin (previous version)
Tested by: nbe@renzel.net (earlier version)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D11780
add support for explicitly requesting that pmap_enter() create a 2MB page
mapping. (Essentially, this feature allows the machine-independent layer to
create superpage mappings preemptively, and not wait for automatic promotion
to occur.)
Export pmap_ps_enabled() to the machine-independent layer.
Add a flag to pmap_pv_insert_pde() that specifies whether it should fail or
reclaim a PV entry when one is not available.
Refactor pmap_enter_pde() into two functions, one by the same name, that is
a general-purpose function for creating PDE PG_PS mappings, and another,
pmap_enter_2mpage(), that is used to prefault 2MB read- and/or execute-only
mappings for execve(2), mmap(2), and shmat(2).
Submitted by: Yufeng Zhou <yz70@rice.edu> (an earlier version)
Reviewed by: kib, markj
Tested by: pho
MFC after: 10 days
Differential Revision: https://reviews.freebsd.org/D11556
--Remove special-case handling of sparc64 bus_dmamap* functions.
Replace with a more generic mechanism that allows MD busdma
implementations to generate inline mapping functions by
defining WANT_INLINE_DMAMAP in <machine/bus_dma.h>. This
is currently useful for sparc64, x86, and arm64, which all
implement non-load dmamap operations as simple wrappers
around map objects which may be bus- or device-specific.
--Remove NULL-checked bus_dmamap macros. Implement the
equivalent NULL checks in the inlined x86 implementation.
For non-x86 platforms, these checks are a minor pessimization
as those platforms do not currently allow NULL maps. NULL
maps were originally allowed on arm64, which appears to have
been the motivation behind adding arm[64]-specific barriers
to bus_dma.h, but that support was removed in r299463.
--Simplify the internal interface used by the bus_dmamap_load*
variants and move it to bus_dma_internal.h
--Fix some drivers that directly include sys/bus_dma.h
despite the recommendations of bus_dma(9)
Reviewed by: kib (previous revision), marius
Differential Revision: https://reviews.freebsd.org/D10729
from machine/proc.h, consistently on all architectures.
Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks
X-Differential revision: https://reviews.freebsd.org/D11080
prevents one from running eg clang built with debug; the new one is
arbitrary (equal to MAXDSIZ) and... well, should be quite future-proof.
Same fix might be applicable to other 64 bit architectures; I'll ask
their respective maintainers to make sure it won't break anything.
Reviewed by: kib
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D10758
in place. To do per-cpu stats, convert all fields that previously were
maintained in the vmmeters that sit in pcpus to counter(9).
- Since some vmmeter stats may be touched at very early stages of boot,
before we have set up UMA and we can do counter_u64_alloc(), provide an
early counter mechanism:
o Leave one spare uint64_t in struct pcpu, named pc_early_dummy_counter.
o Point counter(9) fields of vmmeter to pcpu[0].pc_early_dummy_counter,
so that at early stages of boot, before counters are allocated we already
point to a counter that can be safely written to.
o For sparc64 that required a whole dummy pcpu[MAXCPU] array.
Further related changes:
- Don't include vmmeter.h into pcpu.h.
- vm.stats.vm.v_swappgsout and vm.stats.vm.v_swappgsin changed to 64-bit,
to match kernel representation.
- struct vmmeter hidden under _KERNEL, and only vmstat(1) is an exclusion.
This is based on benno@'s 4-year old patch:
https://lists.freebsd.org/pipermail/freebsd-arch/2013-July/014471.html
Reviewed by: kib, gallatin, marius, lidl
Differential Revision: https://reviews.freebsd.org/D10156
The MFC will include a compat definition of smp_no_rendevous_barrier()
that calls smp_no_rendezvous_barrier().
Reviewed by: gnn, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D10313
I fixed this in 1997, but the fix was over-engineered and fragile and
was broken in 2003 if not before. i386 parameters were copied to 8
other arches verbatim, mostly after they stopped working on i386, and
mostly without the large comment saying how the values were chosen on
i386. powerpc has a non-verbatim copy which just changes the uncritical
parameter and seems to add a sign extension bug to it.
Just treat negative offsets as offsets if they are no more negative than
-db_offset_max (default -64K), and remove all the broken parameters.
-64K is not very negative, but it is enough for frame and stack pointer
offsets since kernel stacks are small.
The over-engineering was mainly to go more negative than -64K for the
negative offset format, without affecting printing for more than a
single address.
Addresses in the top 64K of a (full 32-bit or 64-bit) address space
are now printed less well, but there aren't many interesting ones.
For arches that have many interesting ones very near the top (e.g.,
68k has interrupt vectors there), there would be no good limit for
the negative offset format and -64K is a good as anything.
On the original i386, %dr[4-5] were unimplemented but not very clearly
reserved, so debuggers read them to print them. i386 was still doing
this.
On the original athlon64, %dr[4-5] are documented as reserved but are
aliased to %dr[6-7] unless CR4_DE is set, when accessing them traps.
On 2 of my systems, accessing %dr[4-5] trapped sometimes. On my Haswell
system, the apparent randomness was because the boot CPU starts with
CR4_DE set while all other CPUs start with CR4_DE clear. FreeBSD
doesn't support the data breakpoints enabled by CR4_DE and it never
changes this flag, so the flag remains different across CPUs and
the behaviour seemed inconsistent except while booting when the CPU
doesn't change.
The invalid accesses broke:
- read access for printing the registers in ddb "show watches" on CPUs
with CR4_DE set
- read accesses in fill_dbregs() on CPUs with CR4_DE set. This didn't
implement panic(3) since the user case always skipped %dr[4-5].
- write accesses in set_dbregs(). This also didn't affect userland.
When it didn't trap, the aliasing made it fragile.
Don't print the dummy (zero) values of %dr[4-5] in "show watches" for
i386 or amd64. Fix style bugs near this printing.
amd64 also has space in the dbregs struct for the reserved %dr[8-15]
and already didn't print the dummy values for these, and never accessed
any of the 10 reserved debug registers.
Remove cpufuncs for making the invalid accesses. Even amd64 had these.
Renumber cluase 4 to 3, per what everybody else did when BSD granted
them permission to remove clause 3. My insistance on keeping the same
numbering for legal reasons is too pedantic, so give up on that point.
Submitted by: Jan Schaumann <jschauma@stevens.edu>
Pull Request: https://github.com/freebsd/freebsd/pull/96
PG_PROMOTED, that indicates whether lingering 4KB page mappings might
need to be flushed on a PDE change that restricts or destroys a 2MB
page mapping. This flag allows the pmap to avoid range invalidations
that are both unnecessary and costly.
Reviewed by: kib, markj
MFC after: 6 weeks
Differential Revision: https://reviews.freebsd.org/D9665
Implement get_pcpu() for amd64/sparc64/mips/powerpc, and use it to
replace pcpu_find(curcpu) in MI code.
Reviewed by: andreast, kan, lidl
Tested by: lidl(mips, sparc64), andreast(powerpc)
Differential Revision: https://reviews.freebsd.org/D9587
The switch to get_pcpu() in MI code seems to cause hangs on MIPS.
Back out until we can get a better idea of what's happening there.
Reported by: kan, lidl
SDM states that CLFLUSHOPT instructions can be ordered with other
writes by SFENCE, heavier MFENCE is not required.
Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Previously these were only declared under #ifdef SMP in <machine/smp.h>.
However, these variables are defind in pmap.c unconditionally, and efirt.c
references them unconditionally. This fixes non-SMP kernel builds.
Discussed with: kib
MFC after: 1 week
Reduce the cost of TLB invalidation on x86 by using per-CPU completion flags
Reduce contention during TLB invalidation operations by using a per-CPU
completion flag, rather than a single atomically-updated variable.
On a Westmere system (2 sockets x 4 cores x 1 threads), dtrace measurements
show that smp_tlb_shootdown is about 50% faster with this patch; observations
with VTune show that the percentage of time spent in invlrng_single_page on an
interrupt (actually doing invalidation, rather than synchronization) increases
from 31% with the old mechanism to 71% with the new one. (Running a basic file
server workload.)
Submitted by: Anton Rang <rang at acm.org>
Reviewed by: cem (earlier version)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D8041
Reduce contention during TLB invalidation operations by using a per-CPU
completion flag, rather than a single atomically-updated variable.
On a Westmere system (2 sockets x 4 cores x 1 threads), dtrace measurements
show that smp_tlb_shootdown is about 50% faster with this patch; observations
with VTune show that the percentage of time spent in invlrng_single_page on an
interrupt (actually doing invalidation, rather than synchronization) increases
from 31% with the old mechanism to 71% with the new one. (Running a basic file
server workload.)
Submitted by: Anton Rang <rang at acm.org>
Reviewed by: cem (earlier version), kib
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D8041
Runtime services require special execution environment for the call.
Besides that, OS must inform firmware about runtime virtual memory map
which will be active during the calls, with the SetVirtualAddressMap()
runtime call, done while the 1:1 mapping is still used. There are two
complication: the SetVirtualAddressMap() effectively must be done from
loader, which needs to know kernel address map in advance. More,
despite not explicitely mentioned in the specification, both 1:1 and
the map passed to SetVirtualAddressMap() must be active during the
SetVirtualAddressMap() call. Second, there are buggy BIOSes which
require both mappings active during runtime calls as well, most likely
because they fail to identify all relocations to perform.
On amd64, we can get rid of both problems by providing 1:1 mapping for
the duration of runtime calls, by temprorary remapping user addresses.
As result, we avoid the need for loader to know about future kernel
address map, and avoid bugs in BIOSes. Typically BIOS only maps
something in low 4G. If not runtime bugs, we would take advantage of
the DMAP, as previous versions of this patch did.
Similar but more complicated trick can be used even for i386 and 32bit
runtime, if and when the EFI boot on i386 is supported. We would need
a trampoline page, since potentially whole 4G of VA would be switched
on calls, instead of only userspace portion on amd64.
Context switches are disabled for the duration of the call, FPU access
is granted, and interrupts are not disabled. The later is possible
because kernel is mapped during calls.
To test, the sysctl mib debug.efi_time is provided, setting it to 1
makes one call to EFI get_time() runtime service, on success the efitm
structure is printed to the control terminal. Load efirt.ko, or add
EFIRT option to the kernel config, to enable code.
Discussed with: emaste, imp
Tested by: emaste (mac, qemu)
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Note that lgdt() name is already used for function which, besides
loading GDT, also reloads segment descriptors cache, thus new function
is named bare_lgdt().
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
amd64 pmap.
The new pmap_pinit_pml4() function initializes the level 4 page table
with entries for the kernel mappings. Both functions are needed for
upcoming EFI Runtime Services support.
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
This is not very easy to do, since ddb didn't know when traps are
for single-stepping. It more or less assumed that traps are either
breakpoints or single-step, but even for x86 this became inadequate
with the release of the i386 in ~1986, and FreeBSD passes it other
trap types for NMIs and panics.
On x86, teach ddb when a trap is for single stepping using the %dr6
register. Unknown traps are now treated almost the same as breakpoints
instead of as the same as single-steps. Previously, the classification
of breakpoints was almost correct and everything else was unknown so
had to be treated as a single-step. Now the classification of single-
steps is precise, the classification of breakpoints is almost correct
(as before) and everything else is unknown and treated like a
breakpoint.
This fixes:
- breakpoints not set by ddb, including the main one in kdb_enter(),
were treated as single-steps and not stopped on when stepping
(except for the usual, simple case of a step with residual count 1).
As special cases, kdb_enter() didn't stop for fatal traps or panics
- similarly for "hardware breakpoints".
Use a new MD macro IS_SSTEP_TRAP(type, code) to code to classify
single-steps. This is excessively complicated for bug-for-bug and
backwards compatibilty. Design errors apparently started in Mach
in ~1990 or perhaps in the FreeBSD interface in ~1993. Common trap
types like single steps should have a unique MI code (like the TRAP*
codes for user SIGTRAP) so that debuggers don't need macros like
IS_SSTEP_TRAP() to decode them. But 'type' is actually an ambiguous
MD trap number, and code was always 0 (now it is (int)%dr6 on x86).
So it was impossible to determine the trap type from the args.
Global variables had to be used.
There is already a classification macro db_pc_is_single_step(), but
this just gets in the way. It is only used to recover from bugs in
IS_BREAKPOINT_TRAP(). On some arches, IS_BREAKPOINT_TRAP() just
duplicates the ambiguity in 'type' and misclassifies single-steps as
breakpoints. It defaults to 'false', which is the opposite of what is
needed for bug-for-bug compatibility.
When this is cleaned up, MI classification bits should be passed in
'code'. This could be done now for positive-logic bits, since 'code'
was always 0, but some negative logic is needed for compatibility so
a simple MI classificition is not usable yet.
After reading %dr6, clear the single-step bit in it so that the type
of the next debugger trap can be decoded. This is a little
ddb-specific. ddb doesn't understand the need to clear this bit and
doing it before calling kdb is easiest. gdb would need to reverse
this to support hardware breakpoints, but it just doesn't support
them now since gdbstub doesn't support %dr*.
Fix a bug involving %dr6: when emulating a single-step trap for vm86,
set the bit for it in %dr6. Userland debuggers need this. ddb now
needs this for vm86 bios calls. The bit gets copied to 'code' then
cleared again.
Fix related style bugs:
- when clearing bits for hardware breakpoints in %dr6, spell the mask
as ~0xf on both amd64 and i386 to get the correct number of bits
using sign extension and not need a comment about using the wrong
mask on amd64 (amd64 traps for invalid results but clearing the
reserved top bits didn't trap since they are 0).
- rewrite my old wrong comments about using %dr6 for ddb watchpoints.
The 'cpu' and 'cpu_class' variables were always set to the same value
on amd64 and are legacy holdovers from i386. Remove them entirely on
amd64.
Reviewed by: imp, kib (older version)
Differential Revision: https://reviews.freebsd.org/D7888
The flag specifies that the block which uses FPU must be executed in
critical section, i.e. take no context switches, and does not need an
FPU save area during the execution.
It is intended to be applied around fast and short code pathes where
save area allocation is impossible or undesirable, due to context or
due to the relative cost of calculation vs. allocation.
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
