Re: [PATCH] kvm: don't take vcpu mutex for obviously invalid vcpu ioctls

[Christian Borntraeger]

On 09/22/2014 04:31 PM, Paolo Bonzini wrote:
> 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,

I know, but only for seriously misbehaving userspace, no? See my comment is really a minor one - lets say 2 more cycles for something that exited to userspace - nobody would even notice. I am just disturbed by the fact that we care about something that is not slow-path but broken beyond repair (why does userspace call a non-KVM ioctl on a fd of a vcpu from a different thread (otherwise the mutex would be free)?

Please, can we have an explanation, e.g. something like
"while using trinity to fuzz KVM, we noticed long stalls on invalid ioctls. Lets bail out early on invalid ioctls". or similar?


> depending on what the other VCPU is doing.
> Perhaps something like this would be
> better?
> 
> (Untested, but Tested-by/Reviewed-bys are welcome).

Given that it makes sense to improve a misbehaving userspace, I prefer Davids variant as the patch is smaller and easier to get right. No need to revert, but a better explanation for the use case would be appeciated.

Christian 
> 
> 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) {
> 

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