[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]
Subject: Re: [PATCH v2 0/2] transport-pci: Introduce legacy registers access using AQ
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. > 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? > > > > > > > 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". > > > 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. The mediator can just mirror what guests write over the admin commands. Thanks > -- > MSr >
[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]