Re: [PATCH V2 5/6] virtio: add one field into virtio_device for recording if device uses managed irq

[Ming Lei <ming.lei@redhat.com>]

On Fri, Jul 02, 2021 at 11:55:40AM -0400, Michael S. Tsirkin wrote:
> On Fri, Jul 02, 2021 at 11:05:54PM +0800, Ming Lei wrote:
> > blk-mq needs to know if the device uses managed irq, so add one field
> > to virtio_device for recording if device uses managed irq.
> > 
> > If the driver use managed irq, this flag has to be set so it can be
> > passed to blk-mq.
> > 
> > Cc: "Michael S. Tsirkin" <mst@redhat.com>
> > Cc: Jason Wang <jasowang@redhat.com>
> > Cc: virtualization@lists.linux-foundation.org
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> 
> 
> The API seems somewhat confusing. virtio does not request
> a managed irq as such, does it? I think it's a decision taken
> by the irq core.

vp_request_msix_vectors():

        if (desc) {
                flags |= PCI_IRQ_AFFINITY;
                desc->pre_vectors++; /* virtio config vector */
                vdev->use_managed_irq = true;
        }

        err = pci_alloc_irq_vectors_affinity(vp_dev->pci_dev, nvectors,
                                             nvectors, flags, desc);

Managed irq is used once PCI_IRQ_AFFINITY is passed to
pci_alloc_irq_vectors_affinity().

> 
> Any way to query the irq to find out if it's managed?

So far the managed info isn't exported via /proc/irq, but if one irq is
managed, its smp_affinity & smp_affinity_list attributes can't be
changed successfully.

If irq's debugfs is enabled, this info can be retrieved.


Thanks,
Ming