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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).

Since commit 8b306e2f3c ("KVM: VMX: avoid double list add with VT-d
posted interrupts", 2017-09-27) 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 do not need pre_pcpu since the same commit.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-25-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2021-10-08 19:12:17 -07:00 committed by Paolo Bonzini
parent 74ba5bc872
commit 29802380b6
1 changed files with 3 additions and 20 deletions
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23
arch/x86/kvm/vmx/posted_intr.c
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@ -135,7 +135,6 @@ 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
@ -144,7 +143,6 @@ 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);
@ -156,10 +154,10 @@ 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));
do {
old.control = new.control = pi_desc->control;
@ -168,21 +166,6 @@ int pi_pre_block(struct kvm_vcpu *vcpu)
"Warning: SN field of posted-interrupts "
"is set before blocking\n");
/*
* 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,