Re: [PATCH] KVM: X86: set vcpu preempted only if it is preempted

[Sean Christopherson <seanjc@google.com>]

On Wed, Jan 12, 2022, Paolo Bonzini wrote:
> On 1/12/22 18:30, Sean Christopherson wrote:
> > > Uhhmm, why not? Who says the vcpu will run the moment it becomes
> > > runnable again? Another task could be woken up meanwhile occupying the
> > > real cpu.
> > Hrm, but when emulating HLT, e.g. for an idling vCPU, KVM will voluntarily schedule
> > out the vCPU and mark it as preempted from the guest's perspective.  The vast majority,
> > probably all, usage of steal_time.preempted expects it to truly mean "preempted" as
> > opposed to "not running".
> 
> I'm not sure about that.  In particular, PV TLB shootdown benefits from
> treating a halted vCPU as preempted, because it avoids wakeups of the halted
> vCPUs.

Ah, right.  But that really should be decoupled from steal_time.preempted.  KVM
can technically handle the PV TLB flush any time the vCPU exits, it's just a
question of whether the cost of writing guest memory outweighs the benefits of
potentially avoiding an IPI.  E.g. modifying KVM's fastpath exit loop to toggle
a flag and potentially handle PV TLB flushes is probably a bad idea, but setting
a flag immediately before static_call(kvm_x86_handle_exit)() may be a net win.