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

[Dave Hansen <dave.hansen@intel.com>]

On 3/19/26 23:33, Yi Lai wrote:
> +	/*
> +	 * SYSCALL works differently on FRED, it does not save RIP and RFLAGS
> +	 * to RCX and R11.
> +	 */
> +	unsigned int eax, ebx, ecx, edx;
> +
> +	__cpuid_count(0x7, 0x1, eax, ebx, ecx, edx);
> +	if (!(eax & (1 << 17))) {
> +		/* R11 and EFLAGS should already match. */
> +		assert(ctx->uc_mcontext.gregs[REG_EFL] ==
> +		       ctx->uc_mcontext.gregs[REG_R11]);
> +	}

Could of things:

First, CPUID doesn't tell you if FRED is in use. Is it even on by
default yet? There might not be a better way to do this than checking
CPUID, but checking CPUID is imprecise at best. This at _least_ needs to
be commented. Could you check around for other cases like this in the
x86 selftests and see what the existing solutions are?

Second, this is selftests/, but I feel like we can be a bit more
disciplined than doing raw CPUID calls and sprinkling magic numbers all
about. The least that can be done here is giving bit 17 a name and
making the "is FRED supported" into a helper function.

But it does look like something that needs to get fixed either way!