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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 23/03/2026 8:27 pm, H. Peter Anvin wrote:
> On 2026-03-23 12:17, Andrew Cooper wrote:
>> This doesn't really test whether FRED is active.  It tests whether the
>> OS is not providing strict backwards compatibility, and I think will
>> malfunction when there's a hypervisor above Linux providing strict
>> backwards compatibility.
>>
> But that applies equally to IRET, no? If the hypervisor clobbers the segment
> selector like IRET would in the interest of compatibility then you have the
> same issue.

I suppose.  I for one don't care to provide that level of compatibility.

But for SYSCALL, what are Linux's plans for CRIU or RR ?  I had to fix
SYSCALL legacy behaviour in Xen for the following case:

* PV guest issues SYSCALL on FRED system.  %rcx/%r11 not clobbered
* Migrate to a non-FRED system
* Xen uses a real SYSRET instruction to resume execution


Here, the guest continues executing at whichever dead variable is in %rcx.

CRIU/RR won't be exactly the same, but will suffer the same class of
problem when moving between FRED and non-FRED systems.

~Andrew