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From:   Jason Wang <jasowang@redhat.com>
Date:   Wed, 7 Dec 2022 13:56:42 +0800
Message-ID: <CACGkMEtXiEk72coPnzKmFyTYun_9H+Gm-Hmj68R4x4sb4rp+aw@mail.gmail.com>
Subject: Re: [PATCH v2 01/11] genirq/affinity:: Export irq_create_affinity_masks()
To:     Yongji Xie <xieyongji@bytedance.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        virtualization <virtualization@lists.linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Tue, Dec 6, 2022 at 5:28 PM Yongji Xie <xieyongji@bytedance.com> wrote:
>
> On Tue, Dec 6, 2022 at 4:47 PM Christoph Hellwig <hch@lst.de> wrote:
> >
> > On Tue, Dec 06, 2022 at 04:40:37PM +0800, Yongji Xie wrote:
> > > With the vDPA framework, some drivers (vduse, vdpa-sim) can create
> > > software-defined virtio devices and attach them to the virtio bus.
> > > This kind of virtio device is not a pci device or a platform device.
> > > So it would be needed to export this function if we want to implement
> > > the automatic affinity management for the virtio device driver which
> > > is binded to this device.
> >
> > Why are these devices even using interrupts?
>
> They don't use interrupt. But they use a bound workqueue to run the
> interrupt callback. So the driver needs an algorithm to choose which
> cpu to run the interrupt callback. Then we found the existing
> interrupt affinity spreading mechanism is very suitable for this
> scenario, so we try to export this function to reuse it.
>
> > The whjole vdpa thing
> > is a mess, I also still need to fix up the horrible abuse of the DMA
> > API for something that isn't even DMA, and this just seems to spread
> > that same mistake even further.

I think it's mostly an issue of some vDPA parents, not the vDPA
itself. I had patches to get rid of the DMA API for vDPA simulators.
Will post.

>
> We just want to reuse this algorithm. And it is completely independent
> of the IRQ subsystem. I guess it would not mess things up.

I think so, it's about which CPU do we want to run the callback and
the callback is not necessarily triggered by an IRQ.

Thanks

>
> Thanks,
> Yongji
>