Re: [PATCH v2] selftests/x86: Fix sysret_rip assertion failure on FRED systems

[Andrew Cooper]

On 24/03/2026 8:51 am, Yi Lai wrote:
> The existing 'sysret_rip' selftest asserts that 'regs->r11 ==
> regs->flags'. This check relies on the behavior of the SYSCALL
> instruction on legacy x86_64, which saves 'RFLAGS' into 'R11'.
>
> However, on systems with FRED (Flexible Return and Event Delivery)
> enabled, instead of using registers, all state is saved onto the stack.
> Consequently, 'R11' retains its userspace value, causing the assertion
> to fail.
>
> Fix this by detecting if FRED is enabled and skipping the register
> assertion in that case. The detection is done by checking if the RPL
> bits of the GS selector are preserved after a hardware exception.
> IDT (via IRET) clears the RPL bits of NULL selectors, while FRED (via
> ERETU) preserves them.
>
> Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Yi Lai <yi1.lai@xxxxxxxxx>

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>