Re: [PATCH v7 08/12] KVM/x86/vPMU: Add APIs to support host save/restore the guest lbr stack

[Wei Wang]

On 07/09/2019 08:19 PM, Peter Zijlstra wrote:
> > For the lbr feature, could we thought of it as first come, first served?
> > For example, if we have 2 host threads who want to use lbr at the same time,
> > I think one of them would simply fail to use.
> >
> > So if guest first gets the lbr, host wouldn't take over unless some
> > userspace command (we added to QEMU) is executed to have the vCPU
> > actively stop using lbr.
> Doesn't work that way.
>
> Say you start KVM with LBR emulation, it creates this task event, it
> gets the LBR (nobody else wants it) and the guest works and starts using
> the LBR.
>
> Then the host creates a CPU LBR event and the vCPU suddenly gets denied
> the LBR and the guest no longer functions correctly.
>
> Or you should fail to VMENTER, in which case you starve the guest, but
> at least it doesn't malfunction.

Ok, I think we could go with the first option for now, like this:

When there are other host threads starting to use lbr, host perf notifies
kvm to disable the guest use of lbr (add a notifier_block), which includes:
 - cancel passing through lbr msrs to guest
 - upon guest access to the lbr msr (will trap to KVM now), return #GP
   (or 0).


Best,
Wei