Re: [PATCH v4 11/11] vfio: Disable only uncoordinated discards

[David Hildenbrand <david@redhat.com>]

On 14.01.21 00:57, Alex Williamson wrote:
> On Thu,  7 Jan 2021 14:34:23 +0100
> David Hildenbrand <david@redhat.com> wrote:
> 
>> We support coordinated discarding of RAM using the RamDiscardMgr. Let's
>> unlock support for coordinated discards, keeping uncoordinated discards
>> (e.g., via virtio-balloon) disabled.
>>
>> This unlocks virtio-mem + vfio. Note that vfio used via "nvme://" by the
>> block layer has to be implemented/unlocked separately. For now,
>> virtio-mem only supports x86-64 - spapr IOMMUs are not tested/affected.
> 
> I think SPAPR always uses a vIOMMU, but I won't claim to understand it.

I heard about that, but I also know that they are using virtio-balloon.
Theoretically, virtio-balloon does currently not support vIOMMUs
(iommu=on results in the balloon device essentially being disable).

> Is there anything other than testing that restricts it to x86-64?  ARM

I am not aware, but well, I guess we'll learn as we go/unlock virtio-mem
on these architectures.

Jonathan is trying to enable arm64 support for virtio-mem. I asked him
to test vfio.

I'll have to cleanup my s390x prototype and find a machine to test with
vfio.

> and S390 will use the same type1 IOMMU backend as far as vfio support
> is concerned.  Should we do something more than put it in a commit log
> if we really want to prevent it elsewhere?  Thanks,

I think it should be sufficient to have it in the logs for now. I'll be
involved when unlocking virtio-mem on other archs and will request to
test vfio. In case we find issues, we should try fixing them first
(instead of fencing it / bailing out).

Obviously, this won't stop other RamDiscardMgr users from popping up,
but I guess we'll stumble over that as well.

-- 
Thanks,

David / dhildenb