On Mon, Feb 07 2022, Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote: > On 2/7/22 12:59 PM, Cornelia Huck wrote: >> On Mon, Feb 07 2022, Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote: >> >>> On 2/7/22 3:35 AM, Cornelia Huck wrote: >>>> On Fri, Feb 04 2022, Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote: >>>> >>>>> This was previously removed as unnecessary; while that was true, subsequent >>>>> changes will make KVM an additional required component for vfio-pci-zdev. >>>>> Let's re-introduce CONFIG_VFIO_PCI_ZDEV as now there is actually a reason >>>>> to say 'n' for it (when not planning to CONFIG_KVM). >>>> >>>> Hm... can the file be split into parts that depend on KVM and parts that >>>> don't? Does anybody ever use vfio-pci on a non-kvm s390 system? >>>> >>> >>> It is possible to split out most of the prior CLP/ vfio capability work >>> (but it would not be a totally clean split, zpci_group_cap for example >>> would need to have an inline ifdef since it references a KVM structure) >>> -- I suspect we'll see more of that in the future. >>> I'm not totally sure if there's value in the information being provided >>> today -- this CLP information was all added specifically with >>> userspace->guest delivery in mind. And to answer your other question, >>> I'm not directly aware of non-kvm vfio-pci usage on s390 today; but that >>> doesn't mean there isn't any or won't be in the future of course. With >>> this series, you could CONFIG_KVM=n + CONFIG_VFIO_PCI=y|m and you'll get >>> the standard vfio-pci support but never any vfio-pci-zdev extension. >> >> Yes. Remind me again: if you do standard vfio-pci without the extensions >> grabbing some card-specific information and making them available to the >> guest, you get a working setup, it just always looks like a specific >> card, right? >> > > That's how QEMU treats it anyway, yes. Standard PCI aspects (e.g. > config space) are fine, but for the s390-specific bits we end up making > generalizations / using hard-coded values that are subsequently shared > with the guest when it issues a CLP -- these bits are used to identify > various s390-specific capabilities of the device (an example: based upon > the function type, the guest could derive what format of the function > measurement block can be used. The hard-coded value is otherwise > effectively 'generic device' so use the basic format for this block). > > Basically, we are using vfio to transmit information owned by the host > s390 PCI layer to, ultimately, the guest s390 PCI layer (modified to > reflect what kvm+QEMU supports), so that the guest can treat the device > the same way that the host does. Anything else in between isn't going > to be interested in that information unless it wants to do something > very s390-specific. Thanks, now I remember the details here :) > >>> >>> If we wanted to provide everything we can where KVM isn't strictly >>> required, then let's look at what a split would look like: >>> >>> With or without KVM: >>> zcpi_base_cap >>> zpci_group_cap (with an inline ifdef for KVM [1]) >>> zpci_util_cap >>> zpci_pfip_cap >>> vfio_pci_info_zdev_add_caps >>> vfio_pci_zdev_open (ifdef, just return when !KVM [1]) >>> vfio_pci_zdev_release (ifdef, just return when !KVM [1]) >>> >>> KVM only: >>> vfio_pci_zdev_feat_interp >>> vfio_pci_zdev_feat_aif >>> vfio_pci_zdev_feat_ioat >>> vfio_pci_zdev_group_notifier >>> >>> I suppose such a split has the benefit of flexibility / >>> future-proofing... should a non-kvm use case arrive in the future for >>> s390 and we find we need some s390-specific handling, we're still >>> building vfio-pci-zdev into vfio-pci by default and can just extend that. >>> >>> [1] In this case I would propose renaming CONFIG_VFIO_PCI_ZDEV as we >>> would once again always be building some part of vfio-pci-zdev with >>> vfio-pci on s390 -- maybe something like CONFIG_VFIO_PCI_ZDEV_KVM (wow >>> that's a mouthful) and then use this setting to check "KVM" in my above >>> split. Since this setting will imply PCI, VFIO_PCI and KVM, we can then >>> s/CONFIG_VFIO_PCI_ZDEV/CONFIG_VFIO_PCI_ZDEV_KVM/ for the rest of the >>> series (to continue covering cases like we build KVM but not pci, or not >>> vfio-pci) >>> >>> How does that sound? >> >> Complex :) >> >> I'm not really sure whether it's worth the hassle on an odd chance that >> we may want it for a !KVM usecase in the future (that goes beyond the >> "base" vfio-pci support.) OTOH, it would be cleaner. I'm a bit torn on >> this one. >> > > Well, another option would be to move ahead with this patch as-is, > except to rename s/CONFIG_VFIO_PCI_ZDEV/CONFIG_VFIO_PCI_ZDEV_KVM/ or > something like that (and naturally tweak the title and commit message a > bit). Basically, don't have the name imply a 1:1 relationship with all > of vfio-pci-zdev, even if it will have that effect in practice for now. > > Net result with this series would be we stop building vfio-pci-zdev > without KVM, which means we remove the zdev CLP capabilities when !KVM. > And then if we have a !KVM usecase in the future that needs something > non-standard for s390 (either this CLP info or more likely some other > s390-specific tweak) we can then perform the split, perhaps just as I > describe above. In this way we punt the need for complexity until a > point when (if) we need it, without backing ourselves into a weird case > where we must rename the config parameter (or live with the fact that we > always build some part of vfio-pci-zdev even when CONFIG_VFIO_PCI_ZDEV=n) Ok, I think I like this option the best.
