On Sun, 31 Jan 2021 20:46:40 +0200 Max Gurtovoy <mgurtovoy@xxxxxxxxxx> wrote: > On 1/28/2021 11:02 PM, Alex Williamson wrote: > > On Thu, 28 Jan 2021 17:29:30 +0100 > > Cornelia Huck <cohuck@xxxxxxxxxx> wrote: > > > >> On Tue, 26 Jan 2021 15:27:43 +0200 > >> Max Gurtovoy <mgurtovoy@xxxxxxxxxx> wrote: > >>> On 1/26/2021 5:34 AM, Alex Williamson wrote: > >>>> On Mon, 25 Jan 2021 20:45:22 -0400 > >>>> Jason Gunthorpe <jgg@xxxxxxxxxx> wrote: > >>>> > >>>>> On Mon, Jan 25, 2021 at 04:31:51PM -0700, Alex Williamson wrote: > >>>>>> extensions potentially break vendor drivers, etc. We're only even hand > >>>>>> waving that existing device specific support could be farmed out to new > >>>>>> device specific drivers without even going to the effort to prove that. > >>>>> This is a RFC, not a complete patch series. The RFC is to get feedback > >>>>> on the general design before everyone comits alot of resources and > >>>>> positions get dug in. > >>>>> > >>>>> Do you really think the existing device specific support would be a > >>>>> problem to lift? It already looks pretty clean with the > >>>>> vfio_pci_regops, looks easy enough to lift to the parent. > >>>>> > >>>>>> So far the TODOs rather mask the dirty little secrets of the > >>>>>> extension rather than showing how a vendor derived driver needs to > >>>>>> root around in struct vfio_pci_device to do something useful, so > >>>>>> probably porting actual device specific support rather than further > >>>>>> hand waving would be more helpful. > >>>>> It would be helpful to get actual feedback on the high level design - > >>>>> someting like this was already tried in May and didn't go anywhere - > >>>>> are you surprised that we are reluctant to commit alot of resources > >>>>> doing a complete job just to have it go nowhere again? > >>>> That's not really what I'm getting from your feedback, indicating > >>>> vfio-pci is essentially done, the mlx stub driver should be enough to > >>>> see the direction, and additional concerns can be handled with TODO > >>>> comments. Sorry if this is not construed as actual feedback, I think > >>>> both Connie and I are making an effort to understand this and being > >>>> hampered by lack of a clear api or a vendor driver that's anything more > >>>> than vfio-pci plus an aux bus interface. Thanks, > >>> I think I got the main idea and I'll try to summarize it: > >>> > >>> The separation to vfio-pci.ko and vfio-pci-core.ko is acceptable, and we > >>> do need it to be able to create vendor-vfio-pci.ko driver in the future > >>> to include vendor special souse inside. > >> One other thing I'd like to bring up: What needs to be done in > >> userspace? Does a userspace driver like QEMU need changes to actually > >> exploit this? Does management software like libvirt need to be involved > >> in decision making, or does it just need to provide the knobs to make > >> the driver configurable? > > I'm still pretty nervous about the userspace aspect of this as well. > > QEMU and other actual vfio drivers are probably the least affected, > > at least for QEMU, it'll happily open any device that has a pointer to > > an IOMMU group that's reflected as a vfio group device. Tools like > > libvirt, on the other hand, actually do driver binding and we need to > > consider how they make driver decisions. Jason suggested that the > > vfio-pci driver ought to be only spec compliant behavior, which sounds > > like some deprecation process of splitting out the IGD, NVLink, zpci, > > etc. features into sub-drivers and eventually removing that device > > specific support from vfio-pci. Would we expect libvirt to know, "this > > is an 8086 graphics device, try to bind it to vfio-pci-igd" or "uname > > -m says we're running on s390, try to bind it to vfio-zpci"? Maybe we > > expect derived drivers to only bind to devices they recognize, so > > libvirt could blindly try a whole chain of drivers, ending in vfio-pci. > > Obviously if we have competing drivers that support the same device in > > different ways, that quickly falls apart. > > I think we can leave common arch specific stuff, such as s390 (IIUC) in > the core driver. And only create vfio_pci drivers for > vendor/device/subvendor specific stuff. So on one hand you're telling us that the design principles here can be applied to various other device/platform specific support, but on the other you're saying, but don't do that... > Also, the competing drivers issue can also happen today, right ? after > adding new_id to vfio_pci I don't know how linux will behave if we'll > plug new device with same id to the system. which driver will probe it ? new_id is non-deterministic, that's why we have driver_override. > I don't really afraid of competing drivers since we can ask from vendor > vfio pci_drivers to add vendor_id, device_id, subsystem_vendor and > subsystem_device so we won't have this problem. I don't think that there > will be 2 drivers that drive the same device with these 4 ids. > > Userspace tool can have a map of ids to drivers and bind the device to > the right vfio-pci vendor driver if it has one. if not, bind to vfio_pci.ko. As I've outlined, the support is not really per device, there might be a preferred default driver for the platform, ex. s390. > > Libvirt could also expand its available driver models for the user to > > specify a variant, I'd support that for overriding a choice that libvirt > > might make otherwise, but forcing the user to know this information is > > just passing the buck. > > We can add a code to libvirt as mentioned above. That's rather the question here, what is that algorithm by which a userspace tool such as libvirt would determine the optimal driver for a device? > > Some derived drivers could probably actually include device IDs rather > > than only relying on dynamic ids, but then we get into the problem that > > we're competing with native host driver for a device. The aux bus > > example here is essentially the least troublesome variation since it > > works in conjunction with the native host driver rather than replacing > > it. Thanks, > > same competition after we add new_id to vfio_pci, right ? new_id is already superseded by driver_override to avoid the ambiguity, but to which driver does a userspace tool like libvirt define as the ultimate target driver for a device and how? > A pointer to needed additions to libvirt will be awsome (or any other hint). > > I'll send the V2 soon and then move to libvirt. The libvirt driver for a device likely needs to accept vfio variants and allow users to specify a variant, but the real question is how libvirt makes an educated guess which variant to use initially, which I don't really have any good ideas to resolve. Thanks, Alex
