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KVM: VMX: Remove vCPU from PI wakeup list before updating PID.NV

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Remove the vCPU from the wakeup list before updating the notification
vector in the posted interrupt post-block helper.  There is no need to
wake the current vCPU as it is by definition not blocking.  Practically
speaking this is a nop as it only shaves a few meager cycles in the
unlikely case that the vCPU was migrated and the previous pCPU gets a
wakeup IRQ right before PID.NV is updated.  The real motivation is to
allow for more readable code in the future, when post-block is merged
with vmx_vcpu_pi_load(), at which point removal from the list will be
conditional on the old notification vector.

Opportunistically add comments to document why KVM has a per-CPU spinlock
that, at first glance, appears to be taken only on the owning CPU.
Explicitly call out that the spinlock must be taken with IRQs disabled, a
detail that was "lost" when KVM switched from spin_lock_irqsave() to
spin_lock(), with IRQs disabled for the entirety of the relevant path.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-29-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2021-10-08 19:12:21 -07:00 committed by Paolo Bonzini
parent 724b3962ef
commit baed82c8e4
1 changed files with 22 additions and 5 deletions
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27
arch/x86/kvm/vmx/posted_intr.c
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@ -11,10 +11,22 @@
#include "vmx.h"
/*
* We maintain a per-CPU linked-list of vCPU, so in wakeup_handler() we
* can find which vCPU should be waken up.
* Maintain a per-CPU list of vCPUs that need to be awakened by wakeup_handler()
* when a WAKEUP_VECTOR interrupted is posted. vCPUs are added to the list when
* the vCPU is scheduled out and is blocking (e.g. in HLT) with IRQs enabled.
* The vCPUs posted interrupt descriptor is updated at the same time to set its
* notification vector to WAKEUP_VECTOR, so that posted interrupt from devices
* wake the target vCPUs. vCPUs are removed from the list and the notification
* vector is reset when the vCPU is scheduled in.
*/
static DEFINE_PER_CPU(struct list_head, blocked_vcpu_on_cpu);
/*
* Protect the per-CPU list with a per-CPU spinlock to handle task migration.
* When a blocking vCPU is awakened _and_ migrated to a different pCPU, the
* ->sched_in() path will need to take the vCPU off the list of the _previous_
* CPU. IRQs must be disabled when taking this lock, otherwise deadlock will
* occur if a wakeup IRQ arrives and attempts to acquire the lock.
*/
static DEFINE_PER_CPU(spinlock_t, blocked_vcpu_on_cpu_lock);
static inline struct pi_desc *vcpu_to_pi_desc(struct kvm_vcpu *vcpu)
@ -104,6 +116,14 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
struct pi_desc old, new;
unsigned int dest;
/*
* Remove the vCPU from the wakeup list of the _previous_ pCPU, which
* will not be the same as the current pCPU if the task was migrated.
*/
spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
list_del(&vcpu->blocked_vcpu_list);
spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
dest = cpu_physical_id(vcpu->cpu);
if (!x2apic_mode)
dest = (dest << 8) & 0xFF00;
@ -120,9 +140,6 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
} while (cmpxchg64(&pi_desc->control, old.control,
new.control) != old.control);
spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
list_del(&vcpu->blocked_vcpu_list);
spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
vcpu->pre_pcpu = -1;
}