Re: [RFC 02/10] x86/kvm: Add IBPB support

["Woodhouse, David" <dwmw@amazon.co.uk>]

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On Sat, 2018-01-20 at 12:28 -0800, Liran Alon wrote:
> Isn't it cleaner to check for "boot_cpu_has(X86_FEATURE_IBPB)" both
> in svm_vcpu_init_msrpm() and hardware_setup()?

Strictly speaking that's a different check. That's checking if we're
*using* IBPB, not if it exists.

Now that's probably OK here, since we need it for retpoline *and* IBRS-
based mitigations. And we *might* argue that 'nospectre_v2' on the host
kernel command line should indeed stop us exposing the features to
guests. Maybe.

But next comes IBRS support, and we definitely *won't* want to make
exposing that to guests conditional on X86_FEATURE_IBRS, because in the
retpoline case that won't be set and we probably *will* still want to
expose it to guests based merely on the fact that it exists.

So I think Karim has it right here (modulo the change I already made).

If we want a separate control for "don't expose these to guests", we
should do that explicitly.

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