Re: [PATCH] kvm: don't take vcpu mutex for obviously invalid vcpu ioctls
From: Paolo Bonzini <pbonzini@redhat.com>
Date: 2014-09-22 14:31:51
Also in:
lkml
Subsystem:
kernel virtual machine (kvm), the rest · Maintainers:
Paolo Bonzini, Linus Torvalds
Il 22/09/2014 15:45, Christian Borntraeger ha scritto:
We now have an extra condition check for every valid ioctl, to make an error case go faster. I know, the extra check is just a 1 or 2 cycles if branch prediction is right, but still.
I applied the patch because the delay could be substantial, depending on what the other VCPU is doing. Perhaps something like this would be better? (Untested, but Tested-by/Reviewed-bys are welcome). Paolo
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 84e24b210273..ed31760d79fe 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c@@ -117,12 +117,10 @@ bool kvm_is_mmio_pfn(pfn_t pfn) /* * Switches to specified vcpu, until a matching vcpu_put() */ -int vcpu_load(struct kvm_vcpu *vcpu) +static void __vcpu_load(struct kvm_vcpu *vcpu) { int cpu; - if (mutex_lock_killable(&vcpu->mutex)) - return -EINTR; if (unlikely(vcpu->pid != current->pids[PIDTYPE_PID].pid)) { /* The thread running this VCPU changed. */ struct pid *oldpid = vcpu->pid;
@@ -136,6 +134,14 @@ int vcpu_load(struct kvm_vcpu *vcpu) preempt_notifier_register(&vcpu->preempt_notifier); kvm_arch_vcpu_load(vcpu, cpu); put_cpu(); +} + +int vcpu_load(struct kvm_vcpu *vcpu) +{ + if (mutex_lock_killable(&vcpu->mutex)) + return -EINTR; + + __vcpu_load(vcpu); return 0; }
@@ -1989,9 +1995,6 @@ static long kvm_vcpu_ioctl(struct file *filp, if (vcpu->kvm->mm != current->mm) return -EIO; - if (unlikely(_IOC_TYPE(ioctl) != KVMIO)) - return -EINVAL; - #if defined(CONFIG_S390) || defined(CONFIG_PPC) || defined(CONFIG_MIPS) /* * Special cases: vcpu ioctls that are asynchronous to vcpu execution,
@@ -2001,8 +2004,21 @@ static long kvm_vcpu_ioctl(struct file *filp, return kvm_arch_vcpu_ioctl(filp, ioctl, arg); #endif + if (!mutex_trylock(&vcpu->mutex))) { + /* + * Before a potentially long sleep, check if we'd exit anyway. + * The common case is for the mutex not to be contended, when + * this does not add overhead. + */ + if (unlikely(_IOC_TYPE(ioctl) != KVMIO)) + return -EINVAL; + + if (mutex_lock_killable(&vcpu->mutex)) + return -EINTR; + } + - r = vcpu_load(vcpu); + r = __vcpu_load(vcpu); if (r) return r; switch (ioctl) {