On Tue, Sep 26, 2023, David Woodhouse wrote: > > > On 26 September 2023 19:20:24 CEST, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > >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. > > Thinking about this a bit more, even for HLT it probably isn't just as simple > as checking for mp_state changes. If there's a REQ_EVENT outstanding for > something like a timer delivery, that won't get handled and the IRQ actually > delivered to the local APIC until the vCPU is actually *run*, will it? I haven't been following this conversation, just reacting to seeing "HLT" and "mp_state", which is a bit of a mess: https://lore.kernel.org/all/ZMgIQ5m1jMSAogT4@xxxxxxxxxx
