Re: [PATCH v2 0/3] KVM: EFER.LMSLE cleanup

[Jim Mattson]

On Wed, Sep 21, 2022 at 2:28 AM Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> On Tue, Sep 20, 2022 at 09:36:18PM +0000, Sean Christopherson wrote:
> > Yes, but ideally KVM would explicitly tell the guest "you don't have LMSLE".
> > Probably a moot point, but at the same time I don't see a reason not to be
> > explicit.
>
> Yes but...
>
> On Tue, Sep 20, 2022 at 02:36:34PM -0700, Jim Mattson wrote:
> > Reporting that CPUID bit gives us the right to raise #GP. AMD CPUs
> > (going way back) that don't report EferLmsleUnsupported do not raise
> > #GP.
>
> ... what does "gives us the right" mean exactly?
>
> I'm pretty sure I'm missing something about how KVM works but wouldn't
> it raise a guest #GP when the guest tries to set an unsupported EFER
> bit? I.e., why do you need to explicitly do
>
>         kvm_cpu_cap_set(X86_FEATURE_NO_LMSLE);
>
> and not handle this like any other EFER reserved bit?

EFER.LMLSE is not a reserved bit on AMD64 CPUs, unless
CPUID.80000008:EBX[20] is set (or you're running very, very old
hardware).

We really shouldn't just decide on a whim to treat EFER.LMSLE as
reserved under KVM. The guest CPUID information represents our
detailed contract with the guest software. By setting
CPUID.80000008:EBX[20], we are telling the guest that if it tries to
set EFER.LMSLE, we will raise a #GP.

If we don't set that bit in the guest CPUID information and we raise
#GP on an attempt to set EFER.LMSLE, the virtual hardware is
defective. We could document this behavior as an erratum, but since a
mechanism exists to declare that the guest can expect EFER.LMSLE to
#GP, doesn't it make sense to use it?