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    SubjectRe: [PATCH v3 0/4] KVM: Expose speculation control feature to guests
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    On 30/01/2018 04:00, David Woodhouse wrote:
    > I believe Ashok sent you a change which made us do IBPB on *every*
    > vmexit; I don't think we need that. It's currently done in vcpu_load()
    > which means we'll definitely have done it between running one vCPU and
    > the next, and when vCPUs are pinned we basically never need to do it.
    >
    > We know that VMM (e.g. qemu) userspace could be vulnerable to attacks
    > from guest ring 3, because there is no flush between the vmexit and the
    > host kernel "returning" to the userspace thread. Doing a full IBPB on
    > *every* vmexit would protect from that, but it's overkill. If that's
    > the reason, let's come up with something better.

    Certainly not every vmexit! But doing it on every userspace vmexit and
    every sched_out would not be *that* bad.

    We try really hard to avoid userspace vmexits for everything remotely
    critical to performance (the main exception that's left is the PMTIMER
    I/O port, that Windows likes to access quite a lot), so they shouldn't
    happen that often.

    Paolo

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