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

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Message-ID: <20090430130708.GA13763@redhat.com>
Date:	Thu, 30 Apr 2009 16:07:08 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	davidel@...ilserver.org
Subject: Re: [KVM PATCH v3 2/2] kvm: add support for irqfd via
	eventfd-notification interface

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.

> +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?

> +}
> +

-- 
MST
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