Re: [RFC] KVM: x86: Allow userspace exit on HLT and MWAIT, else yield on MWAIT

[Paolo Bonzini]

On Sat, Sep 23, 2023 at 6:44 PM Alexander Graf <graf@xxxxxxxxx> wrote:
> On 23.09.23 11:24, Paolo Bonzini wrote:
> > Why do you need it?  You can just use KVM_RUN to go to sleep, and if you
> > get another job you kick out the vCPU with pthread_kill.  (I also didn't
> > get the VSM reference).
>
> With the original VSM patches, we used to make a vCPU aware of the fact
> that it can morph into one of many VTLs. That approach turned out to be
> insanely intrusive and fragile and so we're currently reimplementing
> everything as VTLs as vCPUs. That allows us to move the majority of VSM
> functionality to user space. Everything we've seen so far looks as if
> there is no real performance loss with that approach.

Yes, that was also what I remember, sharing the FPU somehow while
having separate vCPU file descriptors.

> One small problem with that is that now user space is responsible for
> switching between VTLs: It determines which VTL is currently running and
> leaves all others (read: all other vCPUs) as stopped. That means if you
> are running happily in KVM_RUN in VTL0 and VTL1 gets an interrupt, user
> space needs to stop VTL0 and unpause VTL1 until it triggers VTL_RETURN
> at which point VTL1 stops execution and VTL0 runs again.

That's with IPIs in VTL1, right? I understand now. My idea was, since
we need a link from VTL1 to VTL0 for the FPU, to use the same link to
trigger a vmexit to userspace if source VTL > destination VTL. I am
not sure how you would handle the case where the destination vCPU is
not running; probably by detecting the IPI when VTL0 restarts on the
destination vCPU?

In any case, making vCPUs poll()-able is sensible.

Paolo

> Nicolas built a patch that exposes "interrupt on vCPU is pending" as an
> ioeventfd user space can request. That way, user space can know whenever
> a currently paused vCPU has a pending interrupt and can act accordingly.
> You could use the same mechanism if you wanted to implement HLT in user
> space, but still use an in-kernel LAPIC.