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Re: [PATCH 8/8] x86-iommu: Fail early if vIOMMU specified after vfio-pci
From: |
Peter Xu |
Subject: |
Re: [PATCH 8/8] x86-iommu: Fail early if vIOMMU specified after vfio-pci |
Date: |
Wed, 27 Oct 2021 16:30:18 +0800 |
On Tue, Oct 26, 2021 at 05:11:39PM +0200, Igor Mammedov wrote:
> On Fri, 22 Oct 2021 10:14:29 +0800
> Peter Xu <peterx@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 <peterx@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/20210906104915.7dd5c934@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.
>
> > 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)
As explained by Alex, I think that's more challenging and it was discussed
quite a few times before, so I didn't mention that in cover letter.
Sorry I should have mentioned some of it.
--
Peter Xu
- Re: [PATCH 7/8] pci: Add pci_for_each_device_all(), (continued)
[PATCH 8/8] x86-iommu: Fail early if vIOMMU specified after vfio-pci, Peter Xu, 2021/10/21