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.
