On Mon, Apr 03, 2023 at 03:16:53PM +0000, Parav Pandit wrote: > > > > From: Michael S. Tsirkin <m...@redhat.com> > > Sent: Monday, April 3, 2023 11:07 AM > > > > > OTOH it is presumably required for scalability anyway, no? > > > No. > > > Most new generation SIOV and SR-IOV devices operate without any para- > > virtualization. > > > > Don't see the connection to PV. You need an emulation layer in the host if > > you > > want to run legacy guests. Looks like it could do transport vq just as well. > > > Transport vq for legacy MMR purpose seems fine with its latency and DMA > overheads. > Your question was about "scalability". > After your latest response, I am unclear what "scalability" means. > Do you mean saving the register space in the PCI device?
yes that's how you used scalability in the past. > If yes, than, no for legacy guests for scalability it is not required, > because the legacy register is subset of 1.x. Weird. what does guest being legacy have to do with a wish to save registers on the host hardware? You don't have so many legacy guests as modern guests? Why? > > > > > And presumably it can all be done in firmware ... > > > > Is there actual hardware that can't implement transport vq but is > > > > going to implement the mmr spec? > > > > > > > Nvidia and Marvell DPUs implement MMR spec. > > > > Hmm implement it in what sense exactly? > > > Do not follow the question. > The proposed series will be implemented as PCI SR-IOV devices using MMR spec. > > > > Transport VQ has very high latency and DMA overheads for 2 to 4 bytes > > read/write. > > > > How many of these 2 byte accesses trigger from a typical guest? > > > Mostly during the VM boot time. 20 to 40 registers read write access. That is not a lot! How long does a DMA operation take then? > > > And before discussing "why not that approach", lets finish reviewing "this > > approach" first. > > > > That's a weird way to put it. We don't want so many ways to do legacy if we > > can > > help it. > Sure, so lets finish the review of current proposal details. > At the moment > a. I don't see any visible gain of transport VQ other than device reset part > I explained. For example, we do not need a new range of device IDs and existing drivers can bind on the host. > b. it can be a way with high latency, DMA overheads on the virtqueue for > read/writes for small access. numbers? -- MST --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscr...@lists.oasis-open.org For additional commands, e-mail: virtio-dev-h...@lists.oasis-open.org
