On Wed, Oct 27, 2021 at 04:30:18PM +0800, Peter Xu wrote: > On Tue, Oct 26, 2021 at 05:11:39PM +0200, Igor Mammedov wrote: > > 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.) > > I see, I didn't try that because I see that q35 even does not have pre_plug() > installed yet, so I need to add it in the same patchset, am I right? > > Frankly I've also no idea why pc has the pre_plug() but not q35.. But if you > think that matters then I can try.
I think I know what I'm missing.. there's the pc_get_hotplug_handler() that conditionally fetch the handler, so when I was trying with q35/VT-d the handler always returned null.. I'll repost with pre_plug(), thanks. -- Peter Xu
