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From:   Yongji Xie <xieyongji@bytedance.com>
Date:   Tue, 6 Dec 2022 17:28:43 +0800
Message-ID: <CACycT3uk4FCswFj8VqqPDgdEM73iqMnYSL8j6DYxCiy1FNSvHQ@mail.gmail.com>
Subject: Re: [PATCH v2 01/11] genirq/affinity:: Export irq_create_affinity_masks()
To:     Christoph Hellwig <hch@lst.de>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@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 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.

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

Thanks,
Yongji