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?
