Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL

[David Woodhouse]

On Wed, 2018-01-31 at 13:18 -0800, Jim Mattson wrote:
> On Wed, Jan 31, 2018 at 12:21 PM, David Woodhouse  wrote:
> 
> > 
> > Reading and writing this MSR is expensive. And if it's yielded to the
> > guest in the MSR bitmap, that means we have to save its value on vmexit
> > and set it back to zero.
>
> Agreed. But my point is that if it's not yielded to the guest in the
> MSR bitmap, then we don't have to save its value on VM-exit and set it
> back to zero. The vmcs02 MSR bitmap is reconstructed on every L1->L2
> transition. Sometimes, it will yield the MSR and sometimes it won't.

Strictly: if SPEC_CTRL is not already set to 1 *and* hasn't been
yielded to the guest in the MSR bitmap, then we don't have to set it
back to zero.

If L1 decides it's *always* going to trap and never pass through, but
the value is already set to non-zero, we need to get that case right.

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