Re: KVM NV + SVE host OS warning

[Oliver Upton <oliver.upton@linux.dev>]

On Tue, Oct 07, 2025 at 11:12:31AM +0000, Jan Kotas wrote:
> Hello,
> 
> I was finally able to do some validation, sorry for a long delay.

No worries, thanks for testing.

> First I applied the "Don't advance PC" patch on top of 6.16.9.
> It fixed the error message, but the Guest didn’t boot.
> I didn’t debug it further.
> 
> Then I applied it on top of 6.17 along with Oliver’s second patch.
> Guest OS stops booting because of an exception, when accessing ZCR_EL2.
> 
> I checked the ESR_EL2 register and it has 0x66000000:
> 
> Access to SVE functionality trapped as a result of CPACR_EL1.ZEN,
> CPTR_EL2.ZEN, CPTR_EL2.TZ, or CPTR_EL3.EZ
> 
> I’ll continue the debug to make sure the issue is not on our end.

Could you please share the repro steps? Also, is the guest kernel
unmodified? FWIW, I tested kvmarm/next as the kernel at all levels,
kvmtool as the VMM and E2H=RES1.

While the trap to L0 is unavoidable, reinjecting the SVE trap depends on
the L0 view of CPTR_EL2 which originates from the in-memory value.
Unless there's a bug lurking this should always be in agreement with the
effective value programmed in CPACR_EL1.

Thanks,
Oliver