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Date: Fri, 13 Mar 2015 14:07:19 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] virtio-pci: Clear IRQ at reset
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, QEMU Developers <qemu-devel@nongnu.org>

On Thu, 03/12 12:15, Michael S. Tsirkin wrote:
> On Thu, Mar 12, 2015 at 11:04:33AM +0000, Peter Maydell wrote:
> > On 12 March 2015 at 10:57, Michael S. Tsirkin <mst@redhat.com> wrote:
> > > This isn't a device reset though.
> > > The function that Fam is touching is called
> > > when a special "virtio reset" register to
> > > poked by the driver.
> > > It only resets part of the device, not all of it,
> > > and it seems reasonable to ask that it clear the
> > > interrupt.
> > 
> > Oh, right, sorry. Yes, that should clear the interrupt, then.
> > (Is there a similar bug on other virtio transports?)
> > 
> > -- PMM
> 
> Hmm interesting.
> 
> I looked at virtio_reset and that one does:
>     vdev->isr = 0;
>     vdev->config_vector = VIRTIO_NO_VECTOR;
>     virtio_notify_vector(vdev, vdev->config_vector);
> 
> which in turn would call
> static void virtio_pci_notify(DeviceState *d, uint16_t vector)
> {
>     VirtIOPCIProxy *proxy = to_virtio_pci_proxy_fast(d);
> 
>     if (msix_enabled(&proxy->pci_dev))
>         msix_notify(&proxy->pci_dev, vector);
>     else {
>         VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
>         pci_set_irq(&proxy->pci_dev, vdev->isr & 1);
>     }
> }
> 
> since isr is 0, and msi is disabled, it looks like
> pci_set_irq will get invoked automatically.
> 
> so at this point I stopped understanding how can
> this patch help.
> 
> Fam, does your patch actually help some guests?
> Could you pls investigate why isn't virtio_reset
> sufficient?

Because virtio_reset doesn't disable msix, so here
"msix_notify(&proxy->pci_dev, 0)" is called.

I tested by applying your patch:

http://www.spinics.net/lists/kernel/msg1943182.html

and adding printf's in QEMU's virtio_reset and virtio_pci_notify.

It shows pci_set_irq is never called.

Fam