Re: [9/8] KVM: x86: limit MSR_IA32_SPEC_CTRL access based on CPUID availability

[David Woodhouse]

On Tue, 2018-01-30 at 08:57 -0800, Jim Mattson wrote:
> It's really hard to tell which patches are being proposed for which
> repositories, but assuming that everything else is correct, I don't
> think your condition is adequate. What if the physical CPU and the
> virtual CPU both have CPUID.(EAX=7H,ECX=0):EDX[26], but only the
> physical CPU has CPUID.(EAX=7H,ECX=0):EDX[27]? If the guest has write
> access to MSR_IA32_SPEC_CTRL, it can set MSR_IA32_SPEC_CTRL[1]
> (STIBP), even though setting that bit in the guest should raise #GP.

Everything we're talking about here is for tip/x86/pti. Which I note
has just updated to be 4.15-based, although I thought it was going to
stay on 4.14 for now. So I've updated my tree at
http://git.infradead.org/linux-retpoline.git/shortlog/refs/heads/ibpb
accordingly.

You can always write to the STIBP bit without a #GP even when it's not
advertised/available.

There's a possibility that we'll want to always trap and *prevent*
that, instead of passing through â because doing so will also have an
effect on the HT siblings. But as discussed, I wanted to get the basics
working before exploring the complex IBRS/STIBP interactions. This much
should be OK to start with.

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