[Devel] [PATCH hci-8.1 v3 0/5] x86/ve: CPUID override follow-ups: arm at once, prefixes, selftests

[Konstantin Khorenko]

VSTOR-148656, every Container start dying with SIGSEGV once
/proc/vz/cpuid_override is non-empty, is fixed in 22.8 by 07338fbae5a3
("x86/traps: emulate CPUID faults for user tasks in
exc_general_protection()"). Working on it turned up two more defects in
the same feature, and the fact that nothing had ever exercised it. This
series takes care of those. None of them is a 22.x regression, all have
been there since the vz9 port.

Patch 2: ve_setup_task() only sets TIF_CPUID_OVERRIDE, the MSR follows
at the next context switch. A task entering a VE through unshare() or
setns() executes CPUID natively until it is scheduled out and sees the
host values. set_cpuid_override() flips the MSR synchronously for the
current task, the way disable_cpuid() does.

Patch 4: check_cpuid_fault() compares two bytes at ip with 0f a2. A
CPUID with a legacy or REX prefix faults the same way, but the handler
sees the prefix and the task gets SIGSEGV. Use the instruction decoder,
as fixup_umip_exception() does, and advance ip by the decoded length.

Patches 1, 3, 5: selftests/ve/ve_cpuid_override_test. Each kernel patch
gets its own case. The test saves and restores the table around every
case and tears down in the parent, because the kselftest harness skips
a child-side teardown after a failed ASSERT.

Tested on sqvm2-4060, the node of the failing CI run (Broadwell VM with
cpuid_fault, cpupools-features 9.1.7, so the table is armed at every
Container start). With only the 22.8 fix, cases 3 and 4 fail the way
their patches describe: host values, and a child killed by signal 11.
With the whole series all four cases pass, repeatedly and on a
non-empty table, and the vzctl-pytest start cases from the original
ticket pass as well.

https://virtuozzo.atlassian.net/browse/VSTOR-148843

Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
---
Changes in v3:
- Patch 1: a saved table the kernel cannot take back in one write (a
  page or more as printed) fails the setup instead of being silently
  left behind by a failed restore; writes replace the table, so there
  is no way to restore it in pieces (Sashiko).
- Patch 1: the child pid lives in the fixture and the parent-side
  teardown kills a child that a failed ASSERT left stopped in the VE
  cgroup, otherwise destroy_ve() fails with EBUSY (Sashiko).
- Patch 1: the Makefile keeps ARCH untouched and decides on a private
  variable, lib.mk picks the clang target by ARCH being empty (Sashiko).
- Link to v2: http://lore.virtuozzo.com/20261006-b4-prep-khorenko-vstor-148656-cpuid-v2-0-f6d1192c7d60@virtuozzo.com

Changes in v2:
- Patch 1: the child stops itself with SIGSTOP after entering the VE
  and the parent resumes it, so faulting is armed by a real context
  switch; the pipe handshake, whose read could return without sleeping,
  is gone (Sashiko).
- Patch 1: setup creates the VE cgroup last, so a failed ASSERT in
  setup leaves nothing behind; a failed restore in teardown is logged
  (Sashiko).
- Patch 1: drop the EAX comparison after the fallback to hardware, the
  child is not pinned and hybrid CPUs may differ per core (Sashiko).
- Patch 1: build the test on x86 only, add the binary to .gitignore
  (Sashiko).
- Patch 3: the no-reschedule case repeats the sequence, a tick can
  still preempt the child in the window; with the handshake gone a
  child that dies before reporting is reaped where the result is read
  and its signal logged (Sashiko).
- Retested on sqvm2-2084 with the whole series: all cases pass and the
  table is restored; on sqvm2-4060 with the stock kernel all cases fail
  with the expected diagnostics.
- Link to v1: http://lore.virtuozzo.com/20261006-b4-prep-khorenko-vstor-148656-cpuid-v1-0-741609f4f03a@virtuozzo.com

---
Konstantin Khorenko (5):
      selftests/ve: check CPUID override emulation for VE tasks
      x86/ve: enable CPUID faulting at once when the current task enters a VE
      selftests/ve: check CPUID override right after entering a VE
      x86/traps: decode the faulting CPUID with the instruction decoder
      selftests/ve: check CPUID override emulation for a prefixed CPUID

 arch/x86/kernel/process.c                          |  12 +
 arch/x86/kernel/traps.c                            |  16 +-
 include/linux/cpuid_override.h                     |   3 +
 kernel/ve/ve.c                                     |   2 +-
 tools/testing/selftests/ve/.gitignore              |   1 +
 tools/testing/selftests/ve/Makefile                |  10 +
 .../testing/selftests/ve/ve_cpuid_override_test.c  | 300 +++++++++++++++++++++
 7 files changed, 337 insertions(+), 7 deletions(-)
---
base-commit: f5d1fdcfffe101fe2bafa7df09d95d2e811c5c5e
change-id: 20261006-b4-prep-khorenko-vstor-148656-cpuid-a5c5993e1f83

Best regards,
--  
Konstantin Khorenko <khorenko at virtuozzo.com>