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linux-kernel@vger.kernel.org On Fri, Jul 07, 2023 at 05:01:38PM +0200, Juergen Gross wrote: > On 07.07.23 16:42, Roger Pau Monné wrote: > > On Fri, Jul 07, 2023 at 04:10:14PM +0200, Juergen Gross wrote: > > > On 07.07.23 11:50, Roger Pau Monné wrote: > > > > On Fri, Jul 07, 2023 at 06:38:48AM +0200, Juergen Gross wrote: > > > > > On 06.07.23 23:49, Stefano Stabellini wrote: > > > > > > On Thu, 6 Jul 2023, Roger Pau Monné wrote: > > > > > > > On Wed, Jul 05, 2023 at 03:41:10PM -0700, Stefano Stabellini wrote: > > > > > > > > On Wed, 5 Jul 2023, Roger Pau Monné wrote: > > > > > > > > > On Tue, Jul 04, 2023 at 08:14:59PM +0300, Oleksandr Tyshchenko wrote: > > > > > > > > > > Part 2 (clarification): > > > > > > > > > > > > > > > > > > > > I think using a special config space register in the root complex would > > > > > > > > > > not be terrible in terms of guest changes because it is easy to > > > > > > > > > > introduce a new root complex driver in Linux and other OSes. The root > > > > > > > > > > complex would still be ECAM compatible so the regular ECAM driver would > > > > > > > > > > still work. A new driver would only be necessary if you want to be able > > > > > > > > > > to access the special config space register. > > > > > > > > > > > > > > > > > > I'm slightly worry of this approach, we end up modifying a root > > > > > > > > > complex emulation in order to avoid modifying a PCI device emulation > > > > > > > > > on QEMU, not sure that's a good trade off. > > > > > > > > > > > > > > > > > > Note also that different architectures will likely have different root > > > > > > > > > complex, and so you might need to modify several of them, plus then > > > > > > > > > arrange the PCI layout correctly in order to have the proper hierarchy > > > > > > > > > so that devices belonging to different driver domains are assigned to > > > > > > > > > different bridges. > > > > > > > > > > > > > > > > I do think that adding something to the PCI conf register somewhere is > > > > > > > > the best option because it is not dependent on ACPI and it is not > > > > > > > > dependent on xenstore both of which are very undesirable. > > > > > > > > > > > > > > > > I am not sure where specifically is the best place. These are 3 ideas > > > > > > > > we came up with: > > > > > > > > 1. PCI root complex > > > > > > > > 2. a register on the device itself > > > > > > > > 3. a new capability of the device > > > > > > > > 4. add one extra dummy PCI device for the sole purpose of exposing the > > > > > > > > grants capability > > > > > > > > > > > > > > > > > > > > > > > > Looking at the spec, there is a way to add a vendor-specific capability > > > > > > > > (cap_vndr = 0x9). Could we use that? It doesn't look like it is used > > > > > > > > today, Linux doesn't parse it. > > > > > > > > > > > > > > I did wonder the same from a quick look at the spec. There's however > > > > > > > a text in the specification that says: > > > > > > > > > > > > > > "The driver SHOULD NOT use the Vendor data capability except for > > > > > > > debugging and reporting purposes." > > > > > > > > > > > > > > So we would at least need to change that because the capability would > > > > > > > then be used by other purposes different than debugging and reporting. > > > > > > > > > > > > > > Seems like a minor adjustment, so might we worth asking upstream about > > > > > > > their opinion, and to get a conversation started. > > > > > > > > > > > > Wait, wouldn't this use-case fall under "reporting" ? It is exactly what > > > > > > we are doing, right? > > > > > > > > > > I'd understand "reporting" as e.g. logging, transferring statistics, ... > > > > > > > > > > We'd like to use it for configuration purposes. > > > > > > > > I've also read it that way. > > > > > > > > > Another idea would be to enhance the virtio IOMMU device to suit our needs: > > > > > we could add the domid as another virtio IOMMU device capability and (for now) > > > > > use bypass mode for all "productive" devices. > > > > > > > > If we have to start adding capabilties, won't it be easier to just add > > > > it to the each device instead of adding it to virtio IOMMU. Or is the > > > > parsing of capabilities device specific, and hence we would have to > > > > implement such parsing for each device? I would expect some > > > > capabilities are shared between all devices, and a Xen capability could > > > > be one of those. > > > > > > Have a look at [1], which is describing the common device config layout. > > > The problem here is that we'd need to add the domid after the queue specific > > > data, resulting in a mess if further queue fields would be added later. > > > > > > We could try that, of course. > > > > Right, we must make it part of the standard if we modify > > virtio_pci_common_cfg, or else newly added fields would overlap the > > Xen specific one. > > > > Would it be possible to signal Xen-grants support in the > > `device_feature` field, and then expose it from a vendor capability? > > IOW, would it be possible to add a Xen-specific hook in the parsing of > > virtio_pci_common_cfg that would then fetch additional data from a > > capability? > > TBH, I don't know. It might require some changes in the central parsing > logic, but this shouldn't be too hard to do. > > > That would likely be less intrusive than adding a new Xen-specific > > field to virtio_pci_common_cfg while still allowing us to do Xen > > specific configuration for all VirtIO devices. > > In case we want to go that route, this should be in a new "platform config" > capability, which might be just another form of a vendor capability. I think telling people that they will need to implement grants-v3 in order to solve this might be too much. I would rather prefer a more concrete solution that doesn't have so many loose ends. Anyway, it's up to the person doing the job, but starting with "you will have to implement grants-v3" is quite likely to deter anyone from attempting to solve this I'm afraid. Thanks, Roger.