Re: [PATCH v15 29/41] KVM: SEV: Synchronize MSR_IA32_XSS from the GHCB when it's valid

John Allen
2025-09-18T22:18:27Z

On Thu, Sep 18, 2025 at 09:42:21PM +0000, Edgecombe, Rick P wrote:
> On Thu, 2025-09-18 at 16:23 -0500, John Allen wrote:
> > The 32bit selftest still doesn't work properly with sev-es, but that was
> > a problem with the previous version too. I suspect there's some
> > incompatibility between sev-es and the test, but I haven't been able to
> > get a good answer on why that might be.
>
> You are talking about test_32bit() in test_shadow_stack.c?

Yes, that's right.

>
> That test relies on a specific CET arch behavior. If you try to transition to a
> 32 bit compatibility mode segment with an SSP with high bits set (outside the 32
> bit address space), a #GP will be triggered by the HW. The test verifies that
> this happens and the kernel handles it appropriately. Could it be platform/mode
> difference and not KVM issue?

I'm fairly certain that this is an issue with any sev-es guest. The
unexpected seg fault happens when we isolate the sigaction32 call used
in the test regardless of shadow stack support. So I wonder if it's
something similar to the case that the test is checking for. Maybe
something to do with the C bit.

Thanks,
John