linux-kernel - Re: [KVM PATCH v3 2/2] kvm: add support for irqfd via eventfd-notification interface

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Message-ID: <49FDCD7C.7070605@redhat.com>
Date:	Sun, 03 May 2009 19:59:40 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Gregory Haskins <ghaskins@...ell.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, davidel@...ilserver.org
Subject: Re: [KVM PATCH v3 2/2] kvm: add support for irqfd via	eventfd-notification
 interface

Michael S. Tsirkin wrote:
> On Mon, Apr 27, 2009 at 02:33:34PM -0400, Gregory Haskins wrote:
>   
>> This allows an eventfd to be registered as an irq source with a guest.  Any
>> signaling operation on the eventfd (via userspace or kernel) will inject
>> the registered GSI at the next available window.
>>
>> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
>>     
>
> If we ever want to use this with e.g. MSI-X emulation in guest, and want
> to be stricly compliant to MSI-X, we'll need a way for guest to mask
> interrupts, and for host to report that a masked interrupt is pending.
> Ideally, all this will be doable with a couple of mmapped pages to avoid
> vmexits/system calls.
>
>   

We could do this in two ways:

- move msix entry emulation into the kernel
- require the device to support replacing its irqfd, and juggle it like so:
   - guest disables msi
       - replace device model fd with eventfd belonging to us
       - when the device fires its eventfd, set the irq pending bit
   - guest enables msi
        - if the pending bit is set, fire the interrupt?
        - replace device model fd with the real irqfd

I'm leaning towards the latter, though it's not an easy call.

>> +static void
>> +irqfd_inject(struct work_struct *work)
>> +{
>> +	struct _irqfd *irqfd = container_of(work, struct _irqfd, work);
>> +	struct kvm *kvm = irqfd->kvm;
>> +
>> +	mutex_lock(&kvm->lock);
>> +	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 1);
>> +	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
>> +	mutex_unlock(&kvm->lock);
>>     
>
> This will do weird stuff (deliver the irq twice) if the irq is
> MSI/MSI-X. I know this was discussed already and is a temporary
> shortcut, but maybe add a comment that we really want kvm_toggle_irq,
> so that we won't forget?
>   

If so, that's a bug.  MSI should ignore kvm_set_irq(..., 0).

-- 
error compiling committee.c: too many arguments to function

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