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Subject: Re: [PATCH v2 0/2] transport-pci: Introduce legacy registers access using AQ
On Wed, May 10, 2023 at 03:01:25PM +0800, Jason Wang wrote: > On Wed, May 10, 2023 at 2:05âPM Michael S. Tsirkin <mst@redhat.com> wrote: > > > > On Mon, May 08, 2023 at 10:23:39AM +0800, Jason Wang wrote: > > > > I thought so too originally. Unfortunately I now think that no, legacy is not > > > > going to be a byproduct of transport virtqueue for modern - > > > > it is different enough that it needs dedicated commands. > > > > > > If you mean the transport virtqueue, I think some dedicated commands > > > for legacy are needed. Then it would be a transport that supports > > > transitional devices. It would be much better than having commands for > > > a partial transport like this patch did. > > > > OK I am beginning to get what you are saying. So your criticism is > > this: what if device supports vq transport for modern, and we want to > > build a transitional device on top. how will that look. yes? > > Yes. I think it needs to be done through the transport virtqueue > otherwise the transport is not self-contained. I mean, any feature can be done over transport vq. But there is value in adding legacy commands to an otherwise modern device without reworking it completely to switch to a different transport. > > A reasonable thing to include at least in the commit log. Parav? > > > > You are also asking what if the device uses transport vq, > > and we want transitional on top of that. > > It's a fair question but I don't exactly get why would > > this legacy support feature be wanted for the vq transport > > and not for other transports. > > Not sure I get the question, but all the existing transport support > legacy, if we want to have another, should the legacy support be a > must or not? This specific proposal is for tunneling legacy over admin vq. It can live alongside a normal modern VF, with hypervisor combining these to create a transitional device. > > > > > > > > > > > > Consider simplest case, multibyte fields. Legacy needs multibyte write, > > > > modern does not even need multibyte read. > > > > > > I'm not sure I will get here, > > > > What does this mean? > > I meant I don't get what the issue if "modern does not even need > multibyte read". parse error again. reword? > > > > > since we can't expose admin vq to > > > guests, it means we need some software mediation. So if we just > > > implement what PCI allows us, then everything would be fine (even if > > > some method is not used). > > > > > > Thanks > > > > To repeat discussion on one of the previous versions, no it will not be > > fine because legacy virtio abuses pci in fundamentally broken ways. > > So yes you need a mediator on the host but even giving this > > mediator a chance to be robust on top of hardware > > means the hardware interface can not simply mirror legacy > > to hardware. > > > > For example, host mediator needs to trap writes into mac, > > buffer them and then send a 6 byte write. > > Now, pci actually does allow 6 byte writes, but legacy > > guests instead to 6 single byte writes for portability reasons. > > It's a known race condition, so PCI over adminq doesn't make it worse. it can however make it better - you can do a single 6 byte write command. > The mediator can just mirror what guests write over the admin > commands. and this "just" just isn't good enough, or we end up with hacks in hardware. > Thanks > > > -- > > MSr > >
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