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

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

On 3/20/26 08:47, Andrew Cooper wrote:
>> 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.
> A reliable way to distinguish IDT and FRED mode is to:
> 
> 1) Load $3 into %fs (x86_64) or %gs (i386) (i.e. whichever isn't thread
> local stoage)
> 2) execute a breakpoint, ignore the signal
> 3) Look to see whether %fs/%gs holds 3 or 0
> 
> IRET has a fun behaviour where it zeroes NULL selectors even if they had
> a non-zero RPL.
> 
> ERETU doesn't do this; Andy Luto and I asked for this minor information
> leak to be removed, and Intel agreed as it served no purpose anyone
> could identify.
> 
> As a consequence, you can use it to determine whether the kernel used
> IRET or ERET to return back to userspace.

I was thinking of just grepping /proc/cpuinfo for "fred", but that
sounds much more fun! :)