Re: [Qemu-devel] [RFC PATCH] vfio: VFIO PCI driver for Qemu

[Avi Kivity]

On 07/26/2012 07:33 PM, Alex Williamson wrote:
>> 
>> In the common case, on x86 (but I'm repeating myself), the iommu group
>> includes just one device, yes?  Could we make pci-stub an alias for the
>> corresponding vfio steps?
> 
> PCI bridges masking devices is not as uncommon as you'd like, that's
> exactly why Andreas is using VFIO instead of KVM assignment.  

Well, we are using it in production for quite a while with few such reports.

> Not to
> mention that VFIO takes a much more strict stance on multifunction ACS
> requirements, typically resulting in all function of a multifunction
> device being inseparable.  So no, I don't think multi-device groups will
> be unusual at all, even on x86.  Playing games with pci-stub sounds like
> a nightmare.  Personally I think we have the opportunity to make libvirt
> and tools like virt-manager a lot better with VFIO.  They no longer need
> to do PCI bridge testing or ACS checking for VFIO and they can better
> inform the user about what devices need to be removed from the host to
> provide safe assignment.
> 
> 
> If we have both vfio and kvm assignment in the same tree there's no
> reason we couldn't intermix them within a VM.  Unfortunately we have to
> beware of KVM assignment's poor ownership model, but that's true whether
> the device is attached to vfio-pci or some other driver.  Maybe we
> should prevent that, but I see that happening by deprecating KVM
> assignment and eventually disabling and removing it.  Thanks,

That's the plan.  By making the command lines compatible, we allow
upgrading the kernel and qemu, but keeping libvirt or another stack, and
more importantly their config files, unchanged.

Perhaps we could do it part-way by making pci-assign do the magic needed
to switch from pci-stub to an iommu group and forwarding the device to
vfio-pci.  But it would probably be root-only.


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