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Date: Fri, 13 Mar 2015 14:28:20 +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 Fri, 03/13 14:07, Fam Zheng wrote:
> 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.

s/0/VIRTIO_NO_VECTOR/

> 
> 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
> 
>