linux-kernel - Re: [KVM PATCH v3 3/3] KVM: Directly inject interrupts if they support lockless operation

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Date:	Wed, 28 Oct 2009 09:20:42 -0400
From:	Gregory Haskins <gregory.haskins@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Gregory Haskins <ghaskins@...ell.com>, kvm@...r.kernel.org,
	alacrityvm-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [KVM PATCH v3 3/3] KVM: Directly inject interrupts if they support
 lockless operation

Michael S. Tsirkin wrote:
> On Tue, Oct 27, 2009 at 02:54:40PM -0400, Gregory Haskins wrote:
>> Michael S. Tsirkin wrote:
>>> On Mon, Oct 26, 2009 at 12:22:08PM -0400, Gregory Haskins wrote:
>>>> IRQFD currently uses a deferred workqueue item to execute the injection
>>>> operation.  It was originally designed this way because kvm_set_irq()
>>>> required the caller to hold the irq_lock mutex, and the eventfd callback
>>>> is invoked from within a non-preemptible critical section.
>>>>
>>>> With the advent of lockless injection support for certain GSIs, the
>>>> deferment mechanism is no longer technically needed in all cases.
>>>> Since context switching to the workqueue is a source of interrupt
>>>> latency, lets switch to a direct method whenever possible.  Fortunately
>>>> for us, the most common use of irqfd (MSI-based GSIs) readily support
>>>> lockless injection.
>>>>
>>>> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
>>> This is a useful optimization I think.
>>> Some comments below.
>>>
>>>> ---
>>>>
>>>>  virt/kvm/eventfd.c |   31 +++++++++++++++++++++++++++----
>>>>  1 files changed, 27 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
>>>> index 30f70fd..e6cc958 100644
>>>> --- a/virt/kvm/eventfd.c
>>>> +++ b/virt/kvm/eventfd.c
>>>> @@ -51,20 +51,34 @@ struct _irqfd {
>>>>  	wait_queue_t              wait;
>>>>  	struct work_struct        inject;
>>>>  	struct work_struct        shutdown;
>>>> +	void (*execute)(struct _irqfd *);
>>>>  };
>>>>  
>>>>  static struct workqueue_struct *irqfd_cleanup_wq;
>>>>  
>>>>  static void
>>>> -irqfd_inject(struct work_struct *work)
>>>> +irqfd_inject(struct _irqfd *irqfd)
>>>>  {
>>>> -	struct _irqfd *irqfd = container_of(work, struct _irqfd, inject);
>>>>  	struct kvm *kvm = irqfd->kvm;
>>>>  
>>>>  	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 1);
>>>>  	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
>>>>  }
>>>>  
>>>> +static void
>>>> +irqfd_deferred_inject(struct work_struct *work)
>>>> +{
>>>> +	struct _irqfd *irqfd = container_of(work, struct _irqfd, inject);
>>>> +
>>>> +	irqfd_inject(irqfd);
>>>> +}
>>>> +
>>>> +static void
>>>> +irqfd_schedule(struct _irqfd *irqfd)
>>>> +{
>>>> +	schedule_work(&irqfd->inject);
>>>> +}
>>>> +
>>>>  /*
>>>>   * Race-free decouple logic (ordering is critical)
>>>>   */
>>>> @@ -126,7 +140,7 @@ irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
>>>>  
>>>>  	if (flags & POLLIN)
>>>>  		/* An event has been signaled, inject an interrupt */
>>>> -		schedule_work(&irqfd->inject);
>>>> +		irqfd->execute(irqfd);
>>>>  
>>>>  	if (flags & POLLHUP) {
>>>>  		/* The eventfd is closing, detach from KVM */
>>>> @@ -179,7 +193,7 @@ kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
>>>>  	irqfd->kvm = kvm;
>>>>  	irqfd->gsi = gsi;
>>>>  	INIT_LIST_HEAD(&irqfd->list);
>>>> -	INIT_WORK(&irqfd->inject, irqfd_inject);
>>>> +	INIT_WORK(&irqfd->inject, irqfd_deferred_inject);
>>>>  	INIT_WORK(&irqfd->shutdown, irqfd_shutdown);
>>>>  
>>>>  	file = eventfd_fget(fd);
>>>> @@ -209,6 +223,15 @@ kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
>>>>  	list_add_tail(&irqfd->list, &kvm->irqfds.items);
>>>>  	spin_unlock_irq(&kvm->irqfds.lock);
>>>>  
>>>> +	ret = kvm_irq_check_lockless(kvm, gsi);
>>>> +	if (ret < 0)
>>>> +		goto fail;
>>>> +
>>>> +	if (ret)
>>>> +		irqfd->execute = &irqfd_inject;
>>>> +	else
>>>> +		irqfd->execute = &irqfd_schedule;
>>>> +
>>> Can't gsi get converted from lockless to non-lockless
>>> after it's checked (by the routing ioctl)?
>> I think I protect against this in patch 2/3 by ensuring that any vectors
>> that are added have to conform to the same locking rules.  The code
>> doesn't support deleting routes, so we really only need to make sure
>> that new routes do not change.
> 
> What I refer to, is when userspace calls KVM_SET_GSI_ROUTING.
> I don't see how your patch helps here: can't a GSI formerly
> used for MSI become unused, and then reused for non-MSI?
> If not, it's a problem I think, because I think userspace currently does this
> sometimes.

I see your point.  I was thinking vectors could only be added, not
deleted, but I see upon further inspection that is not the case.

> 
>>> Kernel will crash then.
>>>
>>> How about, each time we get event from eventfd, we implement
>>> kvm_irqfd_toggle_lockless, which does a single scan, and returns
>>> true/false status (and I really mean toggle, let's not do set 1 / set 0
>>> as well) telling us whether interrupts could be delivered in a lockless
>>> manner?
>> I am not sure I like this idea in general given that I believe I already
>> handle the error case you are concerned with.
>>
>> However, the concept of providing a "toggle" option so we can avoid
>> scanning the list twice is a good one.  That can be done as a new patch
>> series, but it would be a nice addition.
>>
>> Thanks Michael,
>> -Greg
>>
> 
> 



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