On Mon, May 13, 2019 at 07:07:36PM -0700, Andy Lutomirski wrote: > On Mon, May 13, 2019 at 2:09 PM Liran Alon <liran.alon@xxxxxxxxxx> wrote: > > The hope is that the very vast majority of #VMExit handlers will be > > able to completely run without requiring to switch to full address > > space. Therefore, avoiding the performance hit of (2). > > However, for the very few #VMExits that does require to run in full > > kernel address space, we must first kick the sibling hyperthread > > outside of guest and only then switch to full kernel address space > > and only once all hyperthreads return to KVM address space, then > > allow then to enter into guest. > > What exactly does "kick" mean in this context? It sounds like you're > going to need to be able to kick sibling VMs from extremely atomic > contexts like NMI and MCE. Yeah, doing the full synchronous thing from NMI/MCE context sounds exceedingly dodgy, howver.. Realistically they only need to send an IPI to the other sibling; they don't need to wait for the VMExit to complete or anything else. And that is something we can do from NMI context -- with a bit of care. See also arch_irq_work_raise(); specifically we need to ensure we leave the APIC in an idle state, such that if we interrupted an APIC sequence it will not suddenly fail/violate the APIC write/state etc.
