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Subject[PATCH v2 24/43] KVM: VMX: Drop pointless PI.NDST update when blocking
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Don't update Posted Interrupt's NDST, a.k.a. the target pCPU, in the
pre-block path, as NDST is guaranteed to be up-to-date. The comment
about the vCPU being preempted during the update is simply wrong, as the
update path runs with IRQs disabled (from before snapshotting vcpu->cpu,
until after the update completes).

The vCPU can get preempted _before_ the update starts, but not during.
And if the vCPU is preempted before, vmx_vcpu_pi_load() is responsible
for updating NDST when the vCPU is scheduled back in. In that case, the
check against the wakeup vector in vmx_vcpu_pi_load() cannot be true as
that would require the notification vector to have been set to the wakeup
vector _before_ blocking.

Opportunistically switch to using vcpu->cpu for the list/lock lookups,
which presumably used pre_pcpu only for some phantom preemption logic.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/vmx/posted_intr.c | 23 +++--------------------
1 file changed, 3 insertions(+), 20 deletions(-)

diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
index 1688f8dc535a..239e0e72a0dd 100644
--- a/arch/x86/kvm/vmx/posted_intr.c
+++ b/arch/x86/kvm/vmx/posted_intr.c
@@ -130,7 +130,6 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
* - Store the vCPU to the wakeup list, so when interrupts happen
* we can find the right vCPU to wake up.
* - Change the Posted-interrupt descriptor as below:
- * 'NDST' <-- vcpu->pre_pcpu
* 'NV' <-- POSTED_INTR_WAKEUP_VECTOR
* - If 'ON' is set during this process, which means at least one
* interrupt is posted for this vCPU, we cannot block it, in
@@ -139,7 +138,6 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
*/
int pi_pre_block(struct kvm_vcpu *vcpu)
{
- unsigned int dest;
struct pi_desc old, new;
struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);

@@ -153,10 +151,10 @@ int pi_pre_block(struct kvm_vcpu *vcpu)
local_irq_disable();

vcpu->pre_pcpu = vcpu->cpu;
- spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
+ spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->cpu));
list_add_tail(&vcpu->blocked_vcpu_list,
- &per_cpu(blocked_vcpu_on_cpu, vcpu->pre_pcpu));
- spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
+ &per_cpu(blocked_vcpu_on_cpu, vcpu->cpu));
+ spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->cpu));

WARN(pi_desc->sn == 1,
"Posted Interrupt Suppress Notification set before blocking");
@@ -164,21 +162,6 @@ int pi_pre_block(struct kvm_vcpu *vcpu)
do {
old.control = new.control = pi_desc->control;

- /*
- * Since vCPU can be preempted during this process,
- * vcpu->cpu could be different with pre_pcpu, we
- * need to set pre_pcpu as the destination of wakeup
- * notification event, then we can find the right vCPU
- * to wakeup in wakeup handler if interrupts happen
- * when the vCPU is in blocked state.
- */
- dest = cpu_physical_id(vcpu->pre_pcpu);
-
- if (x2apic_mode)
- new.ndst = dest;
- else
- new.ndst = (dest << 8) & 0xFF00;
-
/* set 'NV' to 'wakeup vector' */
new.nv = POSTED_INTR_WAKEUP_VECTOR;
} while (cmpxchg64(&pi_desc->control, old.control,
--
2.33.0.882.g93a45727a2-goog
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