Re: [PATCH 8/8] x86-iommu: Fail early if vIOMMU specified after vfio-pci

[Igor Mammedov <imammedo@redhat.com>]

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,
>