[PATCH v2 28/43] KVM: VMX: Remove vCPU from PI wakeup list before updating PID.NV
From: Sean Christopherson <seanjc@google.com>
Date: 2021-10-09 02:15:45
Also in:
kvm, kvm-riscv, kvmarm, linux-arm-kernel, linux-riscv, lkml
Subsystem:
kernel virtual machine for x86 (kvm/x86), the rest, x86 architecture (32-bit and 64-bit) · Maintainers:
Sean Christopherson, Paolo Bonzini, Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen
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> --- arch/x86/kvm/vmx/posted_intr.c | 49 +++++++++++++++++++++++----------- 1 file changed, 33 insertions(+), 16 deletions(-)
diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
index 2b2206339174..901b7a5f7777 100644
--- a/arch/x86/kvm/vmx/posted_intr.c
+++ b/arch/x86/kvm/vmx/posted_intr.c@@ -10,10 +10,22 @@ #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)
@@ -101,23 +113,28 @@ static void __pi_post_block(struct kvm_vcpu *vcpu) WARN(pi_desc->nv != POSTED_INTR_WAKEUP_VECTOR, "Wakeup handler not enabled while the vCPU was blocking"); - dest = cpu_physical_id(vcpu->cpu); - if (!x2apic_mode) - dest = (dest << 8) & 0xFF00; - - do { - old.control = new.control = READ_ONCE(pi_desc->control); - - new.ndst = dest; - - /* set 'NV' to 'notification vector' */ - new.nv = POSTED_INTR_VECTOR; - } while (cmpxchg64(&pi_desc->control, old.control, - new.control) != old.control); - + /* + * 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; + + do { + old.control = new.control = READ_ONCE(pi_desc->control); + + new.ndst = dest; + + /* set 'NV' to 'notification vector' */ + new.nv = POSTED_INTR_VECTOR; + } while (cmpxchg64(&pi_desc->control, old.control, + new.control) != old.control); + vcpu->pre_pcpu = -1; }
--
2.33.0.882.g93a45727a2-goog