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Re: [PATCH 8/8] x86-iommu: Fail early if vIOMMU specified after vfio-pci
From: |
Igor Mammedov |
Subject: |
Re: [PATCH 8/8] x86-iommu: Fail early if vIOMMU specified after vfio-pci |
Date: |
Tue, 26 Oct 2021 17:11:39 +0200 |
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.)
> 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,
>
- 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