Re: [PATCH v3 1/7] KVM: SVM: Drop RAX check for SVM instructions from the emulator

[Paolo Bonzini]

Il lun 16 mar 2026, 14:49 Yosry Ahmed <yosry@xxxxxxxxxx> ha scritto:
>
> TL;DR is that the emulator support for VMRUN/VMLOAD/VMSAVE is
> broken/unsupported anyway, beyond checking for intercepts and
> pre-intercept exceptions (well, even that is broken), and the RAX
> check should be after that architecturally.

Agreed about that, just ripping it out completely would be fine. It's
only if the emulation happens that it has to be complete.

> Given that the goal of this series is actually to stop injecting a
> non-architectural #GP when KVM fails to read vmcb12, I didn't really
> want to derail it anymore by making it emulator-focused. We can
> probably do a follow-up on just improving the emulator support for
> these instructions as I mentioned above, as that would take its own
> series AFAICT.

Agreed.

> The only reason this patch exists here is because the buggy RAX check
> in the emulator results in injecting a non-architectural #GP on 52-bit
> MAXPHYADDR platforms (the #NPF scenario described in the changelog).

Yeah, that part was clearly broken when physical address bits broke
AMD's 48-bit limit (and I don't think the details of what is
considered an invalid physical address are part of the architectural
description, for example some SMM ranges are carved out by the
VMRUN/VMLOAD/VMSAVE microcode).

Thanks,

Paolo

> So the check is blocking the goal of the series. I didn't really care
> much whether we fixed it or removed it, but Sean did not want to fix
> it and have the emulator directly peek into vCPU state, so removal it
> was.
>
> I hope this makes things clearer.
>