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

[Jim Mattson]

On Wed, Sep 21, 2022 at 6:54 AM Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> 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...?

Yes, after the revert, KVM will treat the bit as reserved, and it will
synthesize a #GP, *in violation of the architectural specification.*
As I said, we could document this behavior as a KVM erratum.

> > 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.

KVM can't just decide willy nilly to reserve arbitrary bits. If it is
in violation of AMD's architectural specification, the virtual CPU 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?
>
> 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...

I feel the same way. :-(

The two patches after the revert are to amend the contract with the
guest (as expressed by the guest CPUID table) so that the KVM virtual
CPU can raise a #GP on EFER.LMSLE and still conform to the
architectural specification.

>From the APM, volume 2, 4.12.2 Data Limit Checks in 64-bit Mode:

> Data segment limit checking in 64-bit mode is not supported by all processor implementations and has been deprecated. If CPUID Fn8000_0008_EBX[EferLmlseUnsupported](bit 20) = 1, 64-bit mode segment limit checking is not supported and attempting to enable this feature by setting EFER.LMSLE =1 will result in a #GP exception.