[RFC PATCH v8 0/3] Intel FRED architecture support for the sysret_rip selftest

[Ammar Faizi]

Hi,

This is an RFC v8. Based on the x86/cpu branch in the tip tree.

The 'syscall' instruction on the Intel FRED architecture does not
clobber %rcx and %r11. This behavior leads to an assertion failure in
the sysret_rip selftest because it asserts %r11 = %rflags.

In the previous discussion, we agreed that there are two cases for
'syscall':

  A) 'syscall' in a FRED system preserves %rcx and %r11.

  B) 'syscall' in a non-FRED system sets %rcx=%rip and %r11=%rflags.

This series fixes the selftest. Make it work on the Intel FRED
architecture. Also, add more tests to ensure the syscall behavior is
consistent. It must always be (A) or always be (B). Not a mix of them.

See the previous discussion here:
https://lore.kernel.org/lkml/5d4ad3e3-034f-c7da-d141-9c001c2343af@xxxxxxxxx

## Changelog revision

v8:
  - Stop using "+r"(rsp) to avoid the red zone problem because it
    generates the wrong Assembly code (Ammar).
    See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108799

  - Update commit message (Ammar).

v7:
  - Fix comment, REGS_ERROR no longer exists in the enum (Ammar).

  - Update commit message (Ammar).

v6:
  - Move the check-regs assertion in sigusr1() to check_regs_result() (HPA).

  - Add a new test just like sigusr1(), but don't modify REG_RCX and
    REG_R11. This is used to test SYSRET behavior consistency (HPA).

v5:
  - Fix do_syscall() return value (Ammar).

v4:
  - Fix the assertion condition inside the SIGUSR1 handler (Xin Li).

  - Explain the purpose of patch #2 in the commit message (HPA).

  - Update commit message (Ammar).

  - Repeat test_syscall_rcx_r11_consistent() 32 times to be more sure
    that the result is really consistent (Ammar).

v3:
  - Test that we don't get a mix of REGS_SAVED and REGS_SYSRET, which
    is a major part of the point (HPA).

v2:
  - Use "+r"(rsp) as the right way to avoid redzone problems per
    Andrew's comment (HPA).

Co-developed-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
Signed-off-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
Signed-off-by: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx>
---

Ammar Faizi (3):
  selftests/x86: sysret_rip: Handle syscall on the Intel FRED architecture
  selftests/x86: sysret_rip: Add more tests to verify the 'syscall' behavior
  selftests/x86: sysret_rip: Test SYSRET with a signal handler

 tools/testing/selftests/x86/sysret_rip.c | 169 +++++++++++++++++++++--
 1 file changed, 160 insertions(+), 9 deletions(-)


base-commit: e067248949e3de7fbeae812b0ccbbee7a401e7aa
-- 
Ammar Faizi