Date:   Tue, 28 Mar 2023 11:44:12 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yongji Xie <xieyongji@...edance.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 03/11] virtio-vdpa: Support interrupt affinity
 spreading mechanism

On Tue, Mar 28, 2023 at 11:33 AM Yongji Xie <xieyongji@...edance.com> wrote:
>
> On Tue, Mar 28, 2023 at 11:14 AM Jason Wang <jasowang@...hat.com> wrote:
> >
> > On Tue, Mar 28, 2023 at 11:03 AM Yongji Xie <xieyongji@...edance.com> wrote:
> > >
> > > On Fri, Mar 24, 2023 at 2:28 PM Jason Wang <jasowang@...hat.com> wrote:
> > > >
> > > > On Thu, Mar 23, 2023 at 1:31 PM Xie Yongji <xieyongji@...edance.com> wrote:
> > > > >
> > > > > To support interrupt affinity spreading mechanism,
> > > > > this makes use of group_cpus_evenly() to create
> > > > > an irq callback affinity mask for each virtqueue
> > > > > of vdpa device. Then we will unify set_vq_affinity
> > > > > callback to pass the affinity to the vdpa device driver.
> > > > >
> > > > > Signed-off-by: Xie Yongji <xieyongji@...edance.com>
> > > >
> > > > Thinking hard of all the logics, I think I've found something interesting.
> > > >
> > > > Commit ad71473d9c437 ("virtio_blk: use virtio IRQ affinity") tries to
> > > > pass irq_affinity to transport specific find_vqs().  This seems a
> > > > layer violation since driver has no knowledge of
> > > >
> > > > 1) whether or not the callback is based on an IRQ
> > > > 2) whether or not the device is a PCI or not (the details are hided by
> > > > the transport driver)
> > > > 3) how many vectors could be used by a device
> > > >
> > > > This means the driver can't actually pass a real affinity masks so the
> > > > commit passes a zero irq affinity structure as a hint in fact, so the
> > > > PCI layer can build a default affinity based that groups cpus evenly
> > > > based on the number of MSI-X vectors (the core logic is the
> > > > group_cpus_evenly). I think we should fix this by replacing the
> > > > irq_affinity structure with
> > > >
> > > > 1) a boolean like auto_cb_spreading
> > > >
> > > > or
> > > >
> > > > 2) queue to cpu mapping
> > > >
> > >
> > > But only the driver knows which queues are used in the control path
> > > which don't need the automatic irq affinity assignment.
> >
> > Is this knowledge awarded by the transport driver now?
> >
>
> This knowledge is awarded by the device driver rather than the transport driver.
>
> E.g. virtio-scsi uses:
>
>     struct irq_affinity desc = { .pre_vectors = 2 }; // vq0 is control
> queue, vq1 is event queue

Ok, but it only works as a hint, it's not a real affinity. As replied,
we can pass an array of boolean in this case then transport driver
knows it doesn't need to use automatic affinity for the first two
queues.

>
> > E.g virtio-blk uses:
> >
> >         struct irq_affinity desc = { 0, };
> >
> > Atleast we can tell the transport driver which vq requires automatic
> > irq affinity.
> >
>
> I think that is what the current implementation does.
>
> > > So I think the
> > > irq_affinity structure can only be created by device drivers and
> > > passed to the virtio-pci/virtio-vdpa driver.
> >
> > This could be not easy since the driver doesn't even know how many
> > interrupts will be used by the transport driver, so it can't built the
> > actual affinity structure.
> >
>
> The actual affinity mask is built by the transport driver,

For PCI yes, it talks directly to the IRQ subsystems.

> device
> driver only passes a hint on which queues don't need the automatic irq
> affinity assignment.

But not for virtio-vDPA since the IRQ needs to be dealt with by the
parent driver. For our case, it's the VDUSE where it doesn't need IRQ
at all, a queue to cpu mapping is sufficient.

Thanks

>
> Thanks,
> Yongji
>

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