Re: [PATCH RFC] virtio-pci: share config interrupt between virtio devices

Amos Kong
2014-09-01T07:58:02Z

On Mon, Sep 01, 2014 at 09:37:30AM +0300, Michael S. Tsirkin wrote:
>

Hi Michael,

> On Mon, Sep 01, 2014 at 01:41:54PM +0800, Amos Kong wrote:
> > One VM only has 128 msix interrupt, virtio-config interrupt
> > has less workload. This patch shares one normal interrupt

Thanks for your quick reply.

> normal == INT#x? Please don't call it normal. The proper name is
> "legacy INT#x".

OK

> So you are trying to use legacy INT#x at the same time
> with MSI-X? This does not work: the PCI spec says:
> While enabled for MSI or MSI-X
> operation, a function is prohibited from using its INTx# pin (if
> implemented) to request
> service (MSI, MSI-X, and INTx# are mutually exclusive).

It means we can't use interrupt and triggered-interrupt together for
one PCI device. I will study this problem.

> does the patch work for you? If it does it might be a (minor) spec
> violation in kvm.

I did some basic testing (multiple nics, scp, ping, etc), it works.

> Besides, INT#x really leads to terrible performance because
> sharing is forced even if there aren't many devices.
>
> Why do we need INT#x?
> How about setting IRQF_SHARED for the config interrupt
> while using MSI-X?
> You'd have to read ISR to check that the interrupt was
> intended for your device.

I have a draft patch to share one MSI-X for all virtio-config, it has
some problem in hotplugging devices. I will continue this way.

> > for configuration between virtio devices.
> >
> > Signed-off-by: Amos Kong <akong@redhat.com>
> > ---
> > drivers/virtio/virtio_pci.c | 41 ++++++++++++++++-------------------------
> > 1 file changed, 16 insertions(+), 25 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_pci.c b/drivers/virtio/virtio_pci.c
> > index 3d1463c..b1263b3 100644
> > --- a/drivers/virtio/virtio_pci.c
> > +++ b/drivers/virtio/virtio_pci.c
> > @@ -52,6 +52,7 @@ struct virtio_pci_device
> > /* Name strings for interrupts. This size should be enough,
> > * and I'm too lazy to allocate each name separately. */
> > char (*msix_names)[256];
> > + char config_msix_name[256];
> > /* Number of available vectors */
> > unsigned msix_vectors;
> > /* Vectors allocated, excluding per-vq vectors if any */
> > @@ -282,12 +283,6 @@ static void vp_free_vectors(struct virtio_device *vdev)
> > free_cpumask_var(vp_dev->msix_affinity_masks[i]);
> >
> > if (vp_dev->msix_enabled) {
> > - /* Disable the vector used for configuration */
> > - iowrite16(VIRTIO_MSI_NO_VECTOR,
> > - vp_dev->ioaddr + VIRTIO_MSI_CONFIG_VECTOR);
> > - /* Flush the write out to device */
> > - ioread16(vp_dev->ioaddr + VIRTIO_MSI_CONFIG_VECTOR);
> > -
> > pci_disable_msix(vp_dev->pci_dev);
> > vp_dev->msix_enabled = 0;
> > }
> > @@ -339,24 +334,18 @@ static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
> > goto error;
> > vp_dev->msix_enabled = 1;
> >
> > - /* Set the vector used for configuration */
> > - v = vp_dev->msix_used_vectors;
> > - snprintf(vp_dev->msix_names[v], sizeof *vp_dev->msix_names,
> > + /* Set shared IRQ for configuration */
> > + snprintf(vp_dev->config_msix_name, sizeof(*vp_dev->msix_names),
> > "%s-config", name);
> > - err = request_irq(vp_dev->msix_entries[v].vector,
> > - vp_config_changed, 0, vp_dev->msix_names[v],
> > + err = request_irq(vp_dev->pci_dev->irq,
> > + vp_config_changed,
> > + IRQF_SHARED,
> > + vp_dev->config_msix_name,
> > vp_dev);
> > - if (err)
> > - goto error;
> > - ++vp_dev->msix_used_vectors;
> > -
> > - iowrite16(v, vp_dev->ioaddr + VIRTIO_MSI_CONFIG_VECTOR);
> > - /* Verify we had enough resources to assign the vector */
> > - v = ioread16(vp_dev->ioaddr + VIRTIO_MSI_CONFIG_VECTOR);
> > - if (v == VIRTIO_MSI_NO_VECTOR) {
> > - err = -EBUSY;
> > + if (!err)
> > + vp_dev->intx_enabled = 1;
> > + else
> > goto error;
> > - }
> >
> > if (!per_vq_vectors) {
> > /* Shared vector for all VQs */
> > @@ -535,14 +524,16 @@ static int vp_try_to_find_vqs(struct virtio_device *vdev, unsigned nvqs,
> > goto error_request;
> > } else {
> > if (per_vq_vectors) {
> > - /* Best option: one for change interrupt, one per vq. */
> > - nvectors = 1;
> > + /* Best option: one normal interrupt for change,
> > + one msix per vq. */
> > + nvectors = 0;
> > for (i = 0; i < nvqs; ++i)
> > if (callbacks[i])
> > ++nvectors;
> > } else {
> > - /* Second best: one for change, shared for all vqs. */
> > - nvectors = 2;
> > + /* Second best: one normal interrupt for
> > + change, share one msix for all vqs. */
> > + nvectors = 1;
> > }
> >
> > err = vp_request_msix_vectors(vdev, nvectors, per_vq_vectors);
> > --
> > 1.9.3
> --
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--
Amos.