On Fri, 22 Oct 2021 10:14:29 +0800 Peter Xu <pet...@redhat.com> wrote:
> Hi, Alex, > > On Thu, Oct 21, 2021 at 04:30:39PM -0600, Alex Williamson wrote: > > On Thu, 21 Oct 2021 18:42:59 +0800 > > Peter Xu <pet...@redhat.com> wrote: > > > > > Scan the pci bus to make sure there's no vfio-pci device attached before > > > vIOMMU > > > is realized. > > > > Sorry, I'm not onboard with this solution at all. > > > > It would be really useful though if this commit log or a code comment > > described exactly the incompatibility for which vfio-pci devices are > > being called out here. Otherwise I see this as a bit of magic voodoo > > that gets lost in lore and copied elsewhere and we're constantly trying > > to figure out specific incompatibilities when vfio-pci devices are > > trying really hard to be "just another device". > > Sure, I can enrich the commit message. > > > > > I infer from the link of the previous alternate solution that this is > > to do with the fact that vfio devices attach a memory listener to the > > device address space. > > IMHO it's not about the memory listeners, I think that' after vfio detected > some vIOMMU memory regions already, which must be based on an vIOMMU address > space being available. I think the problem is that when realize() vfio-pci we > fetch the dma address space specifically for getting the vfio group, while > that > could happen too early, even before vIOMMU is created. > > > Interestingly that previous cover letter also discusses how vdpa devices > > might have a similar issue, which makes it confusing again that we're > > calling > > out vfio-pci devices by name rather than for a behavior. > > Yes I'll need to see whether this approach will be accepted first. I think > similar thing could help VDPA but it's not required there because VDPA has > already worked around using pci_device_iommu_address_space(). So potentially > the only one to "fix" is the vfio-pci device using along with vIOMMU, when the > device ordering is specified in the wrong order. I'll leave the VDPA problem > to Jason to see whether he prefers keeping current code, or switch to a > simpler > one. That should be after this one. > > > > > If the behavior here is that vfio-pci devices attach a listener to the > > device address space, then that provides a couple possible options. We > > could look for devices that have recorded an interest in their address > > space, such as by setting a flag on PCIDevice when someone calls > > pci_device_iommu_address_space(), where we could walk all devices using > > the code in this series to find a device with such a flag. > > Right, we can set a flag for all the pci devices that needs to consolidate > pci_device_iommu_address_space() result, however then it'll be vfio-pci only > so > far. Btw, I actually proposed similar things two months ago, and I think Igor > showed concern on that flag being vague on meaning: (1) > https://lore.kernel.org/qemu-devel/20210906104915.7dd5c...@redhat.com/ > > > > Does it need to be a pre_plug hook? I thought we might just need a > flag in the > > > pci device classes showing that it should be after vIOMMUs, then in > vIOMMU > > > realize functions we walk pci bus to make sure no such device exist? > > > > > > We could have a base vIOMMU class, then that could be in the realize() > of the > > > common class. > > > > We basically don't know if device needs IOMMU or not and can work > > with/without it just fine. In this case I'd think about IOMMU as board > > feature that morphs PCI buses (some of them) (address space, bus numers, > ...). > > So I don't perceive any iommu flag as a device property at all. > > > > As for realize vs pre_plug, the later is the part of abstract realize > > (see: device_set_realized) and is already used by some PCI infrastructure: > > ex: pcie_cap_slot_pre_plug_cb/spapr_pci_pre_plug > > I still think that flag will work, that flag should only shows "whether this > device needs to be specified earlier than vIOMMU", but I can get the point > from > Igor that it's at least confusing on what does the flag mean. > Meanwhile I > don't think that flag will be required, as this is not the first time we name > a > special device in the code, e.g. pc_machine_device_pre_plug_cb(). > intel_iommu.c has it too upon vfio-pci already on making sure caching-mode=on > in vtd_machine_done_notify_one(). I pointed to specifically to _pre_plug() handler and not as implemented here in realize(). Reasoning behind it is that some_device_realize() should not poke into other devices, while pc_machine_device_pre_plug_cb() is part of machine code can/may legitimately access its child devices and verify/ configure them. (Hence I'd drop my suggested-by with current impl.) > If Igor is okay with adding such a flag for PCIDevice class, I can do that in > the new version. I don't have a strong opinion on this. Also, I've suggested to use pre_plug only as the last resort in case vfio-pci can't be made independent of the order (see [1] for reset time suggestion). So why 'reset' approach didn't work out? (this need to be cover letter/commit message as a reason why we are resorting to a hack) > > Thanks, >
