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Subject: Re: [PATCH v2 1/4] Add virtio Admin Virtqueue
On Mon, Jan 31, 2022 at 06:10:39PM +0100, Cornelia Huck wrote: > On Mon, Jan 31 2022, Halil Pasic <pasic@linux.ibm.com> wrote: > > > On Mon, 31 Jan 2022 09:52:54 -0500 > > "Michael S. Tsirkin" <mst@redhat.com> wrote: > > > >> On Mon, Jan 31, 2022 at 03:26:36PM +0100, Cornelia Huck wrote: > >> > On Mon, Jan 31 2022, "Michael S. Tsirkin" <mst@redhat.com> wrote: > >> > > >> > > On Mon, Jan 31, 2022 at 10:16:43AM +0100, Cornelia Huck wrote: > >> > >> On Sun, Jan 30 2022, "Michael S. Tsirkin" <mst@redhat.com> wrote: > >> > >> > >> > >> > On Sun, Jan 30, 2022 at 05:12:46PM +0200, Max Gurtovoy wrote: > >> > >> >> #define VIRTIO_PCI_CAP_MISC_CFG 10 > >> > >> >> > >> > >> >> and > >> > >> >> > >> > >> >> struct virtio_pci_misc_cfg { > >> > >> >> le16 admin_queue_index; /* read-only for driver */ > >> > >> >> }; > >> > >> >> > >> > >> >> Is agreed by all for V3 ? instead of the net and blk AQ index definitions. > >> > >> > > >> > >> > We need to add it to MMIO and CCW I guess too. > >> > >> > >> > >> That seems ok for pci. > >> > >> > >> > >> For ccw, I'd do something like > >> > >> > >> > >> #define CCW_CMD_READ_MISC_CONF 0x82 > >> > >> > >> > >> struct virtio_misc_conf { > >> > >> be16 admin_queue_index; > >> > >> }; > >> > >> > >> > >> bound to revision 3, which gets a payload data containing the length of > >> > >> this structure (for future expansions). > >> > >> > >> > >> Halil, do you think that would work? > >> > >> > >> > >> For mmio, I'd need to think a bit more. Any mmio experts around? > >> > > > >> > > Not an expert but I think we can rely on a feature > >> > > bit to be acked since admin vq is only needed > >> > > after feature negotiation is complete. > >> > > >> > You mean a register that is valid conditionally? I don't see an easy way > >> > to add some kind of "misc" interface for mmio, unlike for the other > >> > transports. > >> > > >> > So something like: > >> > > >> > AdminQueueIndex/0x0c4/R > >> > If VIRTIO_F_ADMIN_VQ has been negotiated, reading from this register > >> > returns the queue index of the administration virtqueue. > >> > >> No, I mean a register that switches 100+ between device specific > >> and misc space. > >> > > > > Maybe adding a register that tells us where does the "misc config > > start" is another option. I don't think we need an open ended > > device-config in practice. I have no idea if there are any upper limits > > on MMIO address space though. If we are constrained there, the switching > > is certainly more efficient. Otherwise, I think having the misc config > > somewhere after device specific config is simpler. > > I think we first need to agree what the "misc" thing is actually > supposed to be. My idea was that we don't have an unlimited supply of > ccws to use for new features, so introducing one for reading "misc" > configuration would be a way to keep things extensible (it also might > make the config/register space for other transports less cluttered). The > same idea (save on ccws) would apply to the multiplexing "action" ccw I > mentioned in my other mail. > > So, for the case here (simply relaying the location of the admin vq), we > don't really need a "misc" mechanism for pci/mmio, but I'd like to > introduce one for ccw. If we agree that it would be useful for pci/mmio > as well, we should introduce it now. OK, my idea was that it's a config that is just live device config but both device and transport independent. Adding it in the misc structure would allow defining it in a single place. > > > > BTW I don't really like this "misc" as the name. FWIW it is less > > miscellaneous that the device specific config, since it is common > > for all devices. I don't have a good name for it, but I would prefer > > "common" over "misc" > > I'm not sure whether "common" would be more confusing, as pci already > has a "common" config (with things that are covered in the mmio > registers, and via various ccw commands.) Does anyone have a better idea > for a bikeshed colour? I'd say "shared" maybe but unfortunately pci already gained "shared memory" so I think it would be confusing. "transport independent" is too wordy... -- MST
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