[PATCH v3 17/26] KVM: VMX: Wake vCPU when delivering posted IRQ even if vCPU == this vCPU

Sean Christopherson
2021-12-08T01:55:44Z

Drop a check that guards triggering a posted interrupt on the currently
running vCPU, and more importantly guards waking the target vCPU if
triggering a posted interrupt fails because the vCPU isn't IN_GUEST_MODE.
The "do nothing" logic when "vcpu == running_vcpu" works only because KVM
doesn't have a path to ->deliver_posted_interrupt() from asynchronous
context, e.g. if apic_timer_expired() were changed to always go down the
posted interrupt path for APICv, or if the IN_GUEST_MODE check in
kvm_use_posted_timer_interrupt() were dropped, and the hrtimer fired in
kvm_vcpu_block() after the final kvm_vcpu_check_block() check, the vCPU
would be scheduled() out without being awakened, i.e. would "miss" the
timer interrupt.

One could argue that invoking kvm_apic_local_deliver() from (soft) IRQ
context for the current running vCPU should be illegal, but nothing in
KVM actually enforces that rules. There's also no strong obvious benefit
to making such behavior illegal, e.g. checking IN_GUEST_MODE and calling
kvm_vcpu_wake_up() is at worst marginally more costly than querying the
current running vCPU.

Lastly, this aligns the non-nested and nested usage of triggering posted
interrupts, and will allow for additional cleanups.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
---
arch/x86/kvm/vmx/vmx.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index fa90eacbf7e2..0eac98589472 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -3993,8 +3993,7 @@ static int vmx_deliver_posted_interrupt(struct kvm_vcpu *vcpu, int vector)
* guaranteed to see PID.ON=1 and sync the PIR to IRR if triggering a
* posted interrupt "fails" because vcpu->mode != IN_GUEST_MODE.
*/
- if (vcpu != kvm_get_running_vcpu() &&
- !kvm_vcpu_trigger_posted_interrupt(vcpu, false))
+ if (!kvm_vcpu_trigger_posted_interrupt(vcpu, false))
kvm_vcpu_wake_up(vcpu);

return 0;
--
2.34.1.400.ga245620fadb-goog