On Fri, Nov 26 2021, Jason Gunthorpe <jgg@xxxxxxxxxx> wrote: > On Fri, Nov 26, 2021 at 01:56:26PM +0100, Cornelia Huck wrote: >> On Thu, Nov 25 2021, Jason Gunthorpe <jgg@xxxxxxxxxx> wrote: >> >> > On Thu, Nov 25, 2021 at 01:27:12PM +0100, Cornelia Huck wrote: >> >> On Wed, Nov 24 2021, Jason Gunthorpe <jgg@xxxxxxxxxx> wrote: >> >> >> >> > On Wed, Nov 24, 2021 at 05:55:49PM +0100, Cornelia Huck wrote: >> >> >> >> >> What I meant to say: If we give userspace the flexibility to operate >> >> >> this, we also must give different device types some flexibility. While >> >> >> subchannels will follow the general flow, they'll probably condense/omit >> >> >> some steps, as I/O is quite different to PCI there. >> >> > >> >> > I would say no - migration is general, no device type should get to >> >> > violate this spec. Did you have something specific in mind? There is >> >> > very little PCI specific here already >> >> >> >> I'm not really thinking about violating the spec, but more omitting >> >> things that do not really apply to the hardware. For example, it is >> >> really easy to shut up a subchannel, we don't really need to wait until >> >> nothing happens anymore, and it doesn't even have MMIO. >> > >> > I've never really looked closely at the s390 mdev drivers.. >> > >> > What does something like AP even do anyhow? The ioctl handler doesn't >> > do anything, there is no mmap hook, how does the VFIO userspace >> > interact with this thing? >> >> For AP, the magic is in the hardware/firmware; the vfio parts are needed >> to configure what is exposed to a given guest, not for operation. Once >> it is up, the hardware will handle any instructions directly, the >> hypervisor will not see them. (Unfortunately, none of the details have >> public documentation.) I have no idea how this would play with migration. > > That is kind of what I thought.. > > VFIO is all about exposing a device to userspace control, sounds like > the S390 drivers skipped that step. Note that what I wrote above is about AP; CCW does indeed trigger operations like start subchannel from userspace and relays interrupts back to userspace. AP is just very dissimilar to basically anything else. > > KVM is all about taking what userspace can already control and giving > it to a guest, in an accelerated way. > > Making a bypass where a KVM guest has more capability than the user > process because VFIO and KVM have been directly coupled completely > upends the whole logical model. > > As we talked with Intel's wbinvd stuff you should have a mental model > where the VFIO userspace process can do anything the KVM guest can do > via ioctls on the mdev. KVM is just an accelerated way to do that same > stuff. Maybe S390 doesn't implement those ioctls, but they are > logically part of the model. FWIW, AP had been a pain to model in a way that we could hand the devices to the guest; if we are supposed to use vfio for this purpose, the current design is probably the best we can get, at least nobody has been able to come up with a better way to interact with the interfaces that we have. CCW needs a kernel part for translations, as it doesn't have an iommu, and the I/O instructions are of course privileged (but so are the instructions for s390 PCI); I think it is quite close to other devices in other respects, only that it has a more transaction-based model. > So, for the migration doc, imagine some non-accelerated KVM that was > intercepting the guest operations and calling the logical ioctls on > the mdev instead. When we talk about MMIO/PIO/etc it also includes > mdev operation ioctls too, and by extension any ioctl accelerated > inside KVM. I think only AP is the really odd one out here; CCW will likely differ in some details... I just wanted to make sure that this will not run counter to the documentation.
