Re: [virtio-comment] [PATCH V3 RESEND 2/4] Introduce the commands set of the transport vq

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 10, 2022 at 04:49:25PM +0800, Zhu, Lingshan wrote:
> > I meant having MSI vectors and let the virtqueue refer to the MSI vectors.
> > 
> > Current design means for a 1024 virtqueues device to store 1024 MSI entries.
> > With the indirection of the MSI vectors array, the device is free to
> > have 1 to 1024 MSI entries.
> I am not sure I get your points, how can the queues decide the MSI entries,
> it should
> be the driver and the platform set the vq's MSI. The driver has to
> decide/control and handle
> the MSI interrupts.
> 
> IMHO there are two options:
> 1)every virtqueue stores its own MSI vector, per-vq MSI vector can get
> better performance
> for sure. But if there are limited MSI resources, the driver may decide to
> share MSI vectors
> among the vqs(1k queues, 1 to 1024 MSI entries), and there should be proper
> interrupt handlers in the driver.
> 
> 2)add a device scope MSI entry(only for vqs, not config interrupt, because
> we don't want to look into config space every time receive a vq interrupt),
> all interrupts from the device are through this MSI entry,
> this is optional, can help save resource. But this is complex, like first
> set a device scope MSI for all vqs, then set a MSI for vq1, in this case,
> we need vq1 use its own MSI to send interrupts, shadows the device MSI. This
> is complex, maybe not worthy, I think the platform(x86, arm) has enough
> MSI resource and it is just one vector per vq.

Consider a PCI device with subfunctions for example. It maintains
MSI-X vectors in an MSI-X table. vq has to select a vector from
that table. Which one?
1- vq can map to a vector number (this is what PCI)
2- vq can just imply vector number e.g. if vector number == vq number
3- vq can include a copy of vector itself

3 will force us to come up with a way to mask vectors in the transport,
not nice.

-- 
MST