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

[Borislav Petkov]

On Wed, Sep 21, 2022 at 06:45:24AM -0700, Jim Mattson wrote:
> 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.

I understand all that. What I'm asking is, what happens in KVM *after*
your patch 1/3 is applied when a guest tries to set EFER.LMSLE? Does it
#GP or does it allow the WRMSR to succeed? I.e., does KVM check when
reserved bits in that MSR are being set?

By looking at it, there's kvm_enable_efer_bits() so it looks like KVM
does control which bits are allowed to set and which not...?

> 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.

See, this is what I don't get - why is it defective? After the revert,
that bit to KVM is reserved.

> 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?

I don't mind all that and the X86_FEATURE bit and so on - I'm just
trying to ask you guys: what is KVM's behavior when the guest tries to
set a reserved EFER bit.

Maybe I'm not expressing myself precisely enough...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette