Commit Graph

3490 Commits

Author SHA1 Message Date
Warner Losh
c90902d39e bus_child_pnpinfo: fix two stragglers
ddfc9c4c59 was missing changes to two files to complete the
bus_child_pnpinfo_str->bus_child_pnpinfo. This fixes the broken kernel
builds.

Sponsored by:		Netflix
2021-06-22 22:10:31 -06:00
Warner Losh
ddfc9c4c59 newbus: Move from bus_child_{pnpinfo,location}_src to bus_child_{pnpinfo,location} with sbuf
Now that the upper layers all go through a layer to tie into these
information functions that translates an sbuf into char * and len. The
current interface suffers issues of what to do in cases of truncation,
etc. Instead, migrate all these functions to using struct sbuf and these
issues go away. The caller is also in charge of any memory allocation
and/or expansion that's needed during this process.

Create a bus_generic_child_{pnpinfo,location} and make it default. It
just returns success. This is for those busses that have no information
for these items. Migrate the now-empty routines to using this as
appropriate.

Document these new interfaces with man pages, and oversight from before.

Reviewed by:		jhb, bcr
Sponsored by:		Netflix
Differential Revision:	https://reviews.freebsd.org/D29937
2021-06-22 20:52:06 -06:00
Alex Richardson
9bb8a4091c Reduce code duplication in machine/_types.h
Many of these typedefs are the same across all architectures or can
be set based on an architecture-independent compiler-provided macro
(e.g. __SIZEOF_SIZE_T__). These macros have been available since GCC 4.6
and Clang sometime before 3.0 (godbolt.org does not have any older clang
versions installed).

I originally considered using the compiler-provided `__FOO_TYPE__` directly.
However, in order to do so we have to check that those match the previous
typedef exactly (not just that they have the same size) since any change
would be an ABI break. For example, changing `long` to `long long` results
in different C++ name mangling. Additionally, Clang and GCC disagree on
the underlying type for some of (u)int*_fast_t types, so this change
only moves the definitions that are identical across all architectures
and does not touch those types.

This de-deduplication will allow us to have a smaller diff downstream in
CheriBSD: we only have to only change the (u)intptr_t definition in
sys/_types.h in CheriBSD instead of having to change machine/_types.h for
all CHERI-enabled architectures (currently RISC-V, AArch64 and MIPS).

Reviewed By: imp, kib
Differential Revision: https://reviews.freebsd.org/D29895
2021-06-14 16:30:16 +01:00
Justin Hibbits
d7bfb412a7 powerpc64le/pmap: Fix superpage promotions
The page table is always big endian.  Without byte swapping on LE, the
promotion ability checks were invalid, and superpage promotions always
failed.
2021-06-05 21:49:42 -05:00
Leandro Lupori
4a66b8083c powerpc: fix boot on pseries without hugepages
Commit 49c894ddce introduced an issue that prevented pseries boot,
when hugepages were not available to the guest. Now large page
info must be available before moea64_install is called, so this change
moves the code that scans large page sizes before the call.

Reviewed by:	jhibbits (IRC)
Sponsored by:	Instituto de Pesquisas Eldorado (eldorado.org.br)
2021-06-02 16:27:36 -03:00
Justin Hibbits
66b2b71df8 Wrap the default SPE config in its own #define
No functional change.  Cleans up the code a little.
2021-05-29 21:53:26 -05:00
Justin Hibbits
0137a09d6e Clean up spr.h
Remove SPRs for CPUs FreeBSD doesn't run on
Add debug register SPRs from the Freescale EREF
2021-05-29 21:53:16 -05:00
Justin Hibbits
811e645d28 Apply r350463(43ded0a321) to powerpc64 radix pmap
Invalidate the last page of a demoted superpage mapping, instead of the
first page, as it results in slightly more promotions and fewer
failures.  While here, replace 'boolean_t's with 'bool's in
mmu_radix_advise().
2021-05-29 15:14:33 -05:00
Justin Hibbits
09947faee8 Apply r350335(5d18382b72) to powerpc64 radix pmap
Simplify pmap_clear_modify() a bit, by assuming that since the superpage
demotion succeeded, all 4k mappings from it are valid.  Deindent the
surrounding code, as there are no 'else' branches in the code anyway.
2021-05-29 15:14:32 -05:00
Justin Hibbits
b40538683a Apply r355991(50079417a5) to powerpc64
Micro-optimize control flow in _pmap_unwire_ptp(), and eliminate
unnecessary parentheses.
2021-05-29 15:14:26 -05:00
Edward Tomasz Napierala
8dc96b74ed cam: clear on-stack CCBs in last few drivers
This changes ahc(4), ahd(4), hptiop(4), hptnr(4), hptrr(4),
and ps3cdrom(4).

Reviewed By:	imp
Sponsored by:	NetApp, Inc.
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D30305
2021-05-21 08:53:59 +01:00
Marcin Wojtas
240429103c Rename ofwpci.c to ofw_pcib.c
It's a class0 driver that implements some pcib methods and creates
a pci bus as its children.
The "ofw_pci" name will be used by a new driver that will be a subclass
of the pci bus.
No functional changes intended.

Submitted by: Kornel Duleba <mindal@semihalf.com>
Reviewed by: andrew
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D30226
2021-05-20 11:22:25 +02:00
Ceri Davies
c1a148873d sys/*/conf/*, docs: fix links to handbook
While here, fix all links to older en_US.ISO8859-1 documentation
in the src/ tree.

PR:             255026
Reported by:    Michael Büker <freebsd@michael-bueker.de>
Reviewed by:    dbaio
Approved by:    blackend (mentor), re (gjb)
MFC after:      10 days
Differential Revision: https://reviews.freebsd.org/D30265
2021-05-20 09:27:10 +01:00
Justin Hibbits
7ed09a6778 powerpc: Rework IPI message processing
Summary:
There's no need to use a while loop in the IPI handler, the message list
is cached once and processed.  Instead, since the existing code calls
ffs(), sort the handlers, and use a simple 'if' sequence.

Reviewed By: nwhitehorn
Differential Revision: https://reviews.freebsd.org/D30018
2021-05-17 08:26:40 -05:00
Justin Hibbits
9aad27931e powerpc64/radix mmu: Remove dead variable
Remove dead variable from mmu_radix_extract_and_hold().  Based on
r352408 for amd64.
2021-05-17 08:26:39 -05:00
Justin Hibbits
a436e66531 powerpc/radix pmap: Convert stat counters from ulongs to counters
This should help performance a hair, for concurrent stat updates, by
reducing contention on cache lines.
2021-05-10 21:26:14 -05:00
Justin Hibbits
31c3770ee5 powerpc/mmu: Actually use the Radix pmap_align_superpage function
This was missed in the conversion to ifuncs.  It might help improve
promotion rates.
2021-05-10 21:26:14 -05:00
Justin Hibbits
49c894ddce powerpc64: Split out DMAP and non-DMAP implementations of some methods
Summary:
Some methods are split between DMAP and non-DMAP, conditional on
hw_direct_map variable.  Rather than checking this variable every time,
use it to install different functions via IFUNCs.

Reviewed By: luporl
Differential Revision: https://reviews.freebsd.org/D30071
2021-05-05 20:57:33 -05:00
Justin Hibbits
be48fe6000 powerpc/xive: Remove POWER9 DD1 IRQ bits
The OPAL_XIVE_*_VIA_IFW flags are used only for POWER9 DD1, which we
don't support.

Noticed while perusing Linux and skiboot git logs.
2021-05-01 16:18:02 -05:00
Justin Hibbits
a6ca7519f8 powerpc64: Optimize radix trap handling a little more
Summary:
Since PCPU can live in a GPR for a while longer, let it, rather than
re-getting it in yet another register.  MFSPR is an expensive operation,
12 clock latency on POWER9, so the fewer operations we need, the better.

Since the check is tightly coupled to the fetch, by reducing the number
of fetch+check, we reduce the stalls, and improve the performance
marginally.  Buildworld was measured at a ~5-7% improvement on a single
run.

Reviewed By: nwhitehorn
Differential Revision: https://reviews.freebsd.org/D30003
2021-04-30 19:58:11 -05:00
Alfredo Dal'Ava Junior
b8bc6b7954 opal_console: fix serial console output corruption on powerpc64
Adds OPAL_CONSOLE_WRITE error handling and implements a call to
OPAL_CONSOLE_WRITE_BUFFER_SPACE to verify if there's enough space
before writing to console.

This fixes serial port output getting corrupted on fast writes, like
on "dmesg" output.

Tested on Raptor Blackbird running powerpc64 BE kernel

Reviewed by:	luporl
Sponsored by:	Eldorado Reserach Institute (eldorado.org.br)
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D29063
2021-04-16 20:10:09 -03:00
Mark Johnston
dfff37765c Rename struct device to struct _device
types.h defines device_t as a typedef of struct device *.  struct device
is defined in subr_bus.c and almost all of the kernel uses device_t.
The LinuxKPI also defines a struct device, so type confusion can occur.

This causes bugs and ambiguity for debugging tools.  Rename the FreeBSD
struct device to struct _device.

Reviewed by:	gbe (man pages)
Reviewed by:	rpokala, imp, jhb
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D29676
2021-04-12 09:32:30 -04:00
Andrew Turner
5d2d599d3f Create VM_MEMATTR_DEVICE on all architectures
This is intended to be used with memory mapped IO, e.g. from
bus_space_map with no flags, or pmap_mapdev.

Use this new memory type in the map request configured by
resource_init_map_request, and in pciconf.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D29692
2021-04-12 06:15:31 +00:00
Leandro Lupori
28d14569c8 powerpc64: add missing TLB invalidations to radix
Radix MMU code was missing TLB invalidations when some Level 3 PDEs were
modified. This caused TLB multi-hit machine check interrupts when
superpages were enabled.

Reviewed by:		jhibbits
MFC after:		2 weeks
Sponsored by:		Eldorado Research Institute (eldorado.org.br)
Differential Revision:	https://reviews.freebsd.org/D29511
2021-04-06 08:31:44 -03:00
Brandon Bergren
0c9f52d4ce powerpc: Fix programmer's switch driver and add to GENERIC
Older G4 and G3 models have a programmer's switch that can be used to
generate an interrupt to drop into the debugger.

This code hadn't been tested for a long time. It had been broken back
in 2005 in r153050.

Repair and modernize the code and add it to GENERIC.

Reviewed by:	jhibbits (approved w/ removal of unused sc_dev var)
Sponsored by:	Tag1 Consulting, Inc.
Differential Revision:	https://reviews.freebsd.org/D29131
2021-04-05 12:04:12 -05:00
Justin Hibbits
16e549ebe2 Merge the QorIQ GPIO drivers between arm and powerpc
Summary:
They're nearly identical, so don't use two copies.  Merge the newer
driver into the older one, and move it to a common location.

Add the Semihalf and associated copyrights in addition to mine, since
it's a non-trivial amount of code merged.

Reviewed By: mw
Differential Revision: https://reviews.freebsd.org/D29520
2021-04-05 10:35:15 -05:00
Mark Johnston
cb5f8694a5 powernv: Include NUMA locality information in the CPU topology
ULE uses this topology to try and preserve locality when migrating
threads between CPUs and when performing work stealing.  Ensure that on
NUMA systems it will at least take the NUMA topology into account.

Reviewed by:	bdragon, jhibbits (previous version)
Tested by:	bdragon
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D28580
2021-04-01 10:25:57 -04:00
Justin Hibbits
921716186f powerpc/aim: Update timebase directly on resume instead of through platform
This only works on single-CPU G4 systems, and more work is needed for
dual-CPU systems.  That said, platform sleep does not work, and this is
currently only used for PMU-based CPU speed change.

The elimination of the platform_smp_timebase_sync() call is so that the
timebase sync rendezvous can be enhanced to perform better
synchronization, which requires a full rendezvous.  This would be
impossible to do on this single-threaded run.
2021-03-31 13:34:06 -05:00
Justin Hibbits
b6d8f3b517 powerpc/powermac: Constrain 'cpu_sleep()' for AIM to mpc745x
Rename cpu_sleep() to mpc745x_sleep() to denote what it's actually
intended for.  This function is very G4-specific, and will not work on
any other CPU.  This will afterward eliminate a
platform_smp_timebase_sync() call by directly updating the timebase
instead.
2021-03-31 13:34:06 -05:00
Justin Hibbits
a5f07fa0c6 powerpc/pseries: Add new hypercall definition, H_REGISTER_PROC_TBL
This will be used by the Radix MMU on pseries.

MFC after:	1 week
2021-03-30 21:22:21 -05:00
Justin Hibbits
895a22583d [PowerPC] Fix ISA_206 subword atomics
The POWER7 subword atomics were not using the correct instructions for
byte and halfword stores in the atomic_fcmpset code.

This only affects builds with custom CFLAGS that have explicitly enabled
ISA_206_ATOMICS.
2021-03-30 20:23:04 -05:00
Jason A. Harmening
8dc8feb53d Clean up a couple of MD warts in vm_fault_populate():
--Eliminate a big ifdef that encompassed all currently-supported
architectures except mips and powerpc32.  This applied to the case
in which we've allocated a superpage but the pager-populated range
is insufficient for a superpage mapping.  For platforms that don't
support superpages the check should be inexpensive as we shouldn't
get a superpage in the first place.  Make the normal-page fallback
logic identical for all platforms and provide a simple implementation
of pmap_ps_enabled() for MIPS and Book-E/AIM32 powerpc.

--Apply the logic for handling pmap_enter() failure if a superpage
mapping can't be supported due to additional protection policy.
Use KERN_PROTECTION_FAILURE instead of KERN_FAILURE for this case,
and note Intel PKU on amd64 as the first example of such protection
policy.

Reviewed by:	kib, markj, bdragon
Differential Revision:	https://reviews.freebsd.org/D29439
2021-03-30 18:15:55 -07:00
Justin Hibbits
74f6cb0f31 [PowerPC] Remove unused IPI type count tracking.
ipi_msg_count is inaccessible outside this file and is never read.

It was introduced in the original SMP support code in r178628 and was never
actually used anywhere.

Remove it to slightly improve IPI performance.

Submitted by:	jhibbits
MFC after:	1 week
2021-03-30 20:03:06 -05:00
Leandro Lupori
75e67b4920 powerpc64: support superpages on pmap_mincore
Now that superpages for HPT MMU has landed, finish implementation of
pmap_mincore by adding support for superpages.

Submitted by:           Fernando Eckhardt Valle <fernando.valle@eldorado.org.br>
Reviewed by:            bdragon, luporl
MFC after:              1 week
Sponsored by:           Eldorado Research Institute (eldorado.org.br)
Differential Revision:  https://reviews.freebsd.org/D29230
2021-03-30 15:54:01 -03:00
Mitchell Horne
7446b0888d gdb: report specific stop reason for watchpoints
The remote protocol allows for implementations to report more specific
reasons for the break in execution back to the client [1]. This is
entirely optional, so it is only implemented for amd64, arm64, and i386
at the moment.

[1] https://sourceware.org/gdb/current/onlinedocs/gdb/Stop-Reply-Packets.html

Reviewed by:	jhb
MFC after:	3 weeks
Sponsored by:	NetApp, Inc.
Sponsored by:	Klara, Inc.
NetApp PR:	51
Differential Revision:	https://reviews.freebsd.org/D29174
2021-03-30 11:36:41 -03:00
Brandon Bergren
5a08df100b [PowerPC] Fix 32-bit Book-E panic due to pve leak
On an INVARIANTS kernel on 32-bit Book-E, we were panicing when running
the libproc tests. This was caused by extra pv entries being generated
accidentally by the pmap icache invalidation code.

Use the same VA (i.e. 0) when freeing the temporary mapping, instead of
some arbitrary address within the zero page.

Failure to do this was causing kernel-side icache syncing to leak
PVE entries when invalidating icache for a non page-aligned address, which
would later result in pages erroneously showing up as mapped to vm_page.

This bug was introduced in r347354 in 2019.

Reviewed by:	jhibbits (in irc)
Sponsored by:	Tag1 Consulting, Inc.
2021-03-29 18:22:16 -05:00
Alfredo Dal'Ava Junior
befb0817bd powerpc: implement bus_map_resource and bus_unmap_resource DEVMETHODs
Implements bus_map_resource and bus_unmap_resource DEVMETHODs to be
used by powerpc targets. This is identical to the amd64 code.

Required by virtio-modern.

Reviewed by:	bryanv
Sponsored by:	Eldorado Research Institute (eldorado.org.br)
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D28012
2021-03-29 20:28:32 -03:00
Mitchell Horne
9d81dd5404 ddb: replace watchpoint set/clear functions
Use the new kdb variants. Print more specific error messages.

Reviewed by:	jhb, markj
MFC after:	3 weeks
Sponsored by:	NetApp, Inc.
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D29156
2021-03-29 12:05:44 -03:00
Mitchell Horne
763107f26c Introduce kdb-level watchpoint functions
This basically mirrors what already exists in ddb, but provides a
slightly improved interface. It allows the caller to specify the
watchpoint access type, and returns more specific error codes to
differentiate failure cases.

This will be used to support hardware watchpoints in gdb(4).

Stubs are provided for architectures lacking hardware watchpoint logic
(mips, powerpc, riscv), while other architectures are added individually
in follow-up commits.

Reviewed by:	jhb, kib, markj
MFC after:	3 weeks
Sponsored by:	NetApp, Inc.
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D29155
2021-03-29 12:05:43 -03:00
Brandon Bergren
bd94c8ab29 [PowerPC] Fix NUMA checking for powernv
At this point in startup, vm_ndomains has not been initialized. Switch
to checking kenv instead.

Fixes incorrect NUMA information being set on multi-domain systems like
Talos II.

Submitted by:	jhibbits
MFC after:	2 weeks
2021-03-27 20:42:49 -05:00
Justin Hibbits
6a762cfae1 powerpc: Fix powerpcspe WRT FPSCR
Summary:
Since powerpcspe doesn't have a traditional FPU, there's no FPSCR, and
no FPRs.  Attempting to use them triggers an illegal instruction trap.
Fix this unconditional cleanup of FPSCR by conditionalizing it on the
FPU being used in the outgoing thread.

Reviewed By: bdragon
Differential Revision: https://reviews.freebsd.org/D29452
2021-03-27 19:24:59 -05:00
Mitchell Horne
720dc6bcb5 Consolidate machine/endian.h definitions
This change serves two purposes.

First, we take advantage of the compiler provided endian definitions to
eliminate some long-standing duplication between the different versions
of this header. __BYTE_ORDER__ has been defined since GCC 4.6, so there
is no need to rely on platform defaults or e.g. __MIPSEB__ to determine
endianness. A new common sub-header is added, but there should be no
changes to the visibility of these definitions.

Second, this eliminates the hand-rolled __bswapNN() routines, again in
favor of the compiler builtins. This was done already for x86 in
e6ff6154d2. The benefit here is that we no longer have to maintain our
own implementations on each arch, and can instead rely on the compiler
to emit appropriate instructions or libcalls, as available. This should
result in equivalent or better code generation. Notably 32-bit arm will
start using the `rev` instruction for these routines, which is available
on armv6+.

PR:		236920
Reviewed by:	arichardson, imp
Tested by:	bdragon (BE powerpc)
MFC after:	3 weeks
Differential Revision:	https://reviews.freebsd.org/D29012
2021-03-26 19:00:22 -03:00
Leandro Lupori
3d0399c718 [PowerPC64] Clear low-order bits of ARPN
PowerISA 2.07B says that the low-order p-12 bits of the real page number
contained in ARPN and LP fields of a PTE must be 0s and are ignored
by the hardware (Book III-S, 5.7.7.1), where 2^p is the actual page size
in bytes, but we were clearing only the LP field.

This worked on bare metal and QEMU with KVM, that ignore these bits,
but caused a kernel panic on QEMU with TCG, that expects them to be
cleared.

This fixes running FreeBSD with HPT superpages enabled on QEMU
with TCG.

MFC after:	2 weeks
Sponsored by:	Eldorado Research Institute (eldorado.org.br)
2021-03-25 14:01:57 -03:00
Jason A. Harmening
d22883d715 Remove PCPU_INC
e4b8deb222 removed the last in-tree uses of PCPU_INC().  Its
potential benefit is also practically nonexistent.  Non-x86
platforms already implement it as PCPU_ADD(..., 1), and according
to [0] there are no recent x86 processors for which the 'inc'
instruction provides a performance benefit over the equivalent
memory-operand form of the 'add' instruction.  The only remaining
benefit of 'inc' is smaller instruction size, which in this case
is inconsequential given the limited number of per-CPU data consumers.

[0]: https://www.agner.org/optimize/instruction_tables.pdf

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D29308
2021-03-20 19:23:59 -07:00
Warner Losh
ba5de7e930 SPDX: Spell 4 clause BSD license correctly 2021-03-11 14:17:54 -07:00
Brandon Bergren
bad9fa5662 [PowerPC] Fix AP bringup on 32-bit AIM SMP
In r361544, the pmap drivers were converted to ifuncs. When doing so,
this changed the call type of pmap functions to be called via the
secure-plt stubs.

These stubs depend on the TOC base being loaded to r30 to run properly.

On SMP AIM (i.e. a dual processor G4 or running 32-bit on G5), since the
APs were being started up from the reset vector instead of going
through __start, they had never had r30 initialized properly, so when the
cpu_reset code in trap_subr32.S attempted to branch to
pmap_cpu_bootstrap(), it was loading the target from the wrong location.

Ensure r30 is set up directly in the cpu_reset trap code, so we can make
PLT calls as normal.

Fixes boot on my SMP G4.

Reviewed by:	jhibbits
MFC after:	3 days
Sponsored by:	Tag1 Consulting, Inc.
2021-03-06 15:46:28 -06:00
Alfredo Dal'Ava Junior
231633a2e9 [PowerPC64] add mpr to GENERIC64 and GENERIC64LE
Submitted by:	Andre Fernando da Silva <andre.silva@eldorado.org.br>
Reviewed by:	luporl, alfredo, Sreekanth Reddy <sreekanth.reddy@broadcom.com> (by email)
Sponsored by:	Eldorado Research Institute (eldorado.org.br)
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D25785
2021-03-02 22:21:43 -03:00
Brandon Bergren
5001c579ba [PowerPC64LE] pseries: Fix input buffering logic.
In uart_phyp_get(), when the internal buffer is empty, we make a
hypercall to retrieve up to 16 bytes of input data from the
hypervisor. As this is specified to be returned in BE format, we need
to do a 64-bit byte swap on the first and second half of the data.

If the buffer being passed in was insufficient to return the fetched
data, we store the remainder in the internal buffer and use it to
satisfy the following calls to uart_phyp_get() until it is drained.

However, in this case, we were accidentally byteswapping the internal
buffer again.

Move the byteswapping code to just after the hypercall so it only gets
swapped when we're filling the buffer.

Fixes arrow keys in qemu on pseries, among other console oddities.

Sponsored by:	Tag1 Consulting, Inc.
MFC after:	3 days
2021-02-25 14:50:13 -06:00
John Baldwin
67932460c7 Add a VA_IS_CLEANMAP() macro.
This macro returns true if a provided virtual address is contained
in the kernel's clean submap.

In CHERI kernels, the buffer cache and transient I/O map are allocated
as separate regions.  Abstracting this check reduces the diff relative
to FreeBSD.  It is perhaps slightly more readable as well.

Reviewed by:	kib
Obtained from:	CheriBSD
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D28710
2021-02-17 16:32:11 -08:00
Brandon Bergren
d26f2a50ff powerpc64: Fix boot on virtual-mode OF (PowerMac G5)
In 78599c32ef, CFI endproc decoration was
added to locore64.S. However, it missed the subtle detail that
__restartkernel_virtual() falls through to __restartkernel(). This was
causing boot failure on PowerMac G5, as it tried to execute the
epilogue as code.

Fix this by branching to __restartkernel() instead of intentionally
running off the end of the function.

While here, add some additional notes on how the virtual mode restart
works.

MFC after:	3 days
2021-02-07 16:13:55 -06:00