Re: [Qemu-devel] [PATCH RFC 0/4] intel_iommu: Do sanity check of vfio-pci earlier

[Paolo Bonzini <pbonzini@redhat.com>]

On 21/08/19 07:03, Peter Xu wrote:
> On Tue, Aug 20, 2019 at 08:24:49AM +0200, Paolo Bonzini wrote:
>> On 20/08/19 07:22, Peter Xu wrote:
>>> On Mon, Aug 12, 2019 at 09:45:27AM +0200, Peter Xu wrote:
>>>> This is a RFC series.
>>>>
>>>> The VT-d code has some defects, one of them is that we cannot detect
>>>> the misuse of vIOMMU and vfio-pci early enough.
>>>>
>>>> For example, logically this is not allowed:
>>>>
>>>>   -device intel-iommu,caching-mode=off \
>>>>   -device vfio-pci,host=05:00.0
>>>>
>>>> Because the caching mode is required to make vfio-pci devices
>>>> functional.
>>>>
>>>> Previously we did this sanity check in vtd_iommu_notify_flag_changed()
>>>> as when the memory regions change their attributes.  However that's
>>>> too late in most cases!  Because the memory region layouts will only
>>>> change after IOMMU is enabled, and that's in most cases during the
>>>> guest OS boots.  So when the configuration is wrong, we will only bail
>>>> out during the guest boots rather than simply telling the user before
>>>> QEMU starts.
>>>>
>>>> The same problem happens on device hotplug, say, when we have this:
>>>>
>>>>   -device intel-iommu,caching-mode=off
>>>>
>>>> Then we do something like:
>>>>
>>>>   (HMP) device_add vfio-pci,host=05:00.0,bus=pcie.1
>>>>
>>>> If at that time the vIOMMU is enabled in the guest then the QEMU
>>>> process will simply quit directly due to this hotplug event.  This is
>>>> a bit insane...
>>>>
>>>> This series tries to solve above two problems by introducing two
>>>> sanity checks upon these places separately:
>>>>
>>>>   - machine done
>>>>   - hotplug device
>>>>
>>>> This is a bit awkward but I hope this could be better than before.
>>>> There is of course other solutions like hard-code the check into
>>>> vfio-pci but I feel it even more unpretty.  I didn't think out any
>>>> better way to do this, if there is please kindly shout out.
>>>>
>>>> Please have a look to see whether this would be acceptable, thanks.
>>>
>>> Any more comment on this?
>>
>> No problem from me, but I wouldn't mind if someone else merged it. :)
> 
> Can I read this as an "acked-by"? :)

Yes, it shouldn't even need Acked-by since there are other maintainers
that handle this part of the tree:

Paolo Bonzini <pbonzini@redhat.com> (maintainer:X86 TCG CPUs)
Richard Henderson <rth@twiddle.net> (maintainer:X86 TCG CPUs)
Eduardo Habkost <ehabkost@redhat.com> (maintainer:X86 TCG CPUs)
"Michael S. Tsirkin" <mst@redhat.com> (supporter:PC)
Marcel Apfelbaum <marcel.apfelbaum@gmail.com> (supporter:PC)

> Michael, should this be for your tree?  What do you think about the
> series?  Please let me know what I need to do to move this forward.  I
> can repost a non-rfc series if needed, but it'll be exactly the same
> content.
> 
> Regards,
>