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Subject: Re: [PATCH v2 0/2] transport-pci: Introduce legacy registers access using AQ
On Wed, May 10, 2023 at 09:08:44PM +0000, Parav Pandit wrote: > Hi Jason, Michel, > > > From: virtio-comment@lists.oasis-open.org <virtio-comment@lists.oasis- > > open.org> On Behalf Of Parav Pandit > > Sent: Wednesday, May 10, 2023 1:34 PM > > > > > From: Michael S. Tsirkin <mst@redhat.com> > > > Sent: Wednesday, May 10, 2023 12:16 PM > > > > > > On Wed, May 10, 2023 at 12:11:50PM -0400, Parav Pandit wrote: > > > > > > > > > > > > On 5/10/2023 2:04 AM, Michael S. Tsirkin 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? > > > > > A reasonable thing to include at least in the commit log. Parav? > > > > > > > > > I am still trying to understand what is "vq transport for modern"? > > > > Do you mean transporting currently defined config space access over vq? > > > > If so, is this VQ belong to the guest or hypervisor? > > > > > > https://lore.kernel.org/all/20220826100034.200432-2- > > > lingshan.zhu%40intel.com/t.mbox.gz > > > > The gz link is not accessible. > > But I got the right link [1]. > > > > [1] https://lore.kernel.org/all/20220826100034.200432-2- > > lingshan.zhu@intel.com/ > > > > 1. Above patch cover letter [1] is missing the basic objective/problem > > statement. > > i.e. why a transport virtqueue is needed? > > But I probably get the idea of [1] as we did the AQ. > > > > 2. Commit log says about > > a. querying resource of management device (aka group owner in AQ now) b. > > creating and destroying the managed device (aka group owner creating group > > member devices) c. configure the managed device (aka group owner > > configuring/composing group member devices such as VFs, SFs, SIOV). > > > > So, all above 2.a to 2.c belongs to the admin group owner and group > > management commands like how it is defined in the AQ proposal. > > > > So, 3 out of the 4 motivations are achieved by AQ proposal. > > This AQ belongs to the hypervisor. I am clear on this part. > > > > 4th point in cover letter is: "config virtqueues of the managed device". > > > > This work belongs to the driver -> device direct communication using a queue > > from driver to device. > > So, I imagine this work can be done using a queue by the guest driver and > > serviced by the device like how a guest driver configures the queue today > > without any mediation. > > For PCI, MMIO transport, surely this can be done by the PCI device directly > > being is PF, VF or SIOV. > > (Instead of config register, using a new queue interface). Looks fine to me. > > > > Can this new cfg queue mediated like CVQ that is done in a sw? May be yes. > > Should it be always mediated when it is of VF, SIOV Device? Mostly no because > > it is yet another VQ for PF, VF, SIOV. > > > > I am yet to parse rest of the 4 patches, please give me some time to review it. > > I went over the past work of [1], [2]. > > [1] https://lore.kernel.org/all/20220826100034.200432-2-lingshan.zhu@intel.com/ > [2] https://lists.oasis-open.org/archives/virtio-comment/202208/msg00141.html > > The "virtio q as transport" in [2] is bit misleading as its only role is to transport the _registers_ of the SIOV_R1 device through its parent PF. > Rest of the work is the pure management work to manage the life cycle of SIOV devices (create/delete/configure/compose). > > And the motivation is also clear is to provide composing a virtio device for the guest VM for the backward compatibility for 1.x part of the specification. > > It seems fine and indeed orthogonal to me that: it is for backward compatibility for already defined config fields for existing guest VM driver. > > It does not conflict with the cfgq/cmdq idea whose main purpose is for the new config fields, new use cases that doesn't require any mediation. > Such cfgq works across PF, VF, SF/SIOV devices in uniform way without mediation. > It also scales device register memory also very well in predictable way. > > The registers and feature bits access described in [2], can certainly be done using new non_legacy new AQ commands. > Both legacy and non-legacy command have different semantics as Michael mentioned. > > The AQ semantics that Michael did as opposed to "virtqueue as transport" fits well for the use case described in [1]. > > There are changes on going in MSI-X area and SIOV, so you might want to wait for it. > Or proposed command in [1] should be tagged as siov_r1, then things will be cleaner. > > With that I don't see legacy 3 commands anyway conflict with [1]. > Some commands functionality is common between [1] and this proposal. > But that's how the legacy is. It is confined to legacy emulation. > So [1] can be done as follow_on to AQ and these series. > > A small note about [2]. > virtio_transportq_ctrl_dev_attribute should be detached from CREATE call and split to two commands. > So that VF and SF/SIOV can both utilize it. > SF/SIOV_R1 can use the creation and config part. > VFs will use only the device features + config space. I think some kind of "enable" command for VFs might have value. No? -- MST
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