Re: [PATCH v4 0/7] vdpa: decouple reset of iotlb mapping from device reset

[Si-Wei Liu <si-wei.liu@oracle.com>]

Thanks a lot for testing! Please be aware that there's a follow-up fix 
for a potential oops in this v4 series:

https://lore.kernel.org/virtualization/1698102863-21122-1-git-send-email-si-wei.liu@oracle.com/

Would be nice to have it applied for any tests.

Thanks,
-Siwei

On 10/23/2023 11:51 PM, Lei Yang wrote:
> QE tested this series v4 with regression testing on real nic, there is
> no new regression bug.
>
> Tested-by: Lei Yang <leiyang@redhat.com>
>
> On Tue, Oct 24, 2023 at 6:02 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
>>
>>
>> On 10/22/2023 8:51 PM, Jason Wang wrote:
>>> Hi Si-Wei:
>>>
>>> On Sat, Oct 21, 2023 at 5:28 PM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
>>>> In order to reduce needlessly high setup and teardown cost
>>>> of iotlb mapping during live migration, it's crucial to
>>>> decouple the vhost-vdpa iotlb abstraction from the virtio
>>>> device life cycle, i.e. iotlb mappings should be left
>>>> intact across virtio device reset [1]. For it to work, the
>>>> on-chip IOMMU parent device could implement a separate
>>>> .reset_map() operation callback to restore 1:1 DMA mapping
>>>> without having to resort to the .reset() callback, the
>>>> latter of which is mainly used to reset virtio device state.
>>>> This new .reset_map() callback will be invoked only before
>>>> the vhost-vdpa driver is to be removed and detached from
>>>> the vdpa bus, such that other vdpa bus drivers, e.g.
>>>> virtio-vdpa, can start with 1:1 DMA mapping when they
>>>> are attached. For the context, those on-chip IOMMU parent
>>>> devices, create the 1:1 DMA mapping at vdpa device creation,
>>>> and they would implicitly destroy the 1:1 mapping when
>>>> the first .set_map or .dma_map callback is invoked.
>>>>
>>>> This patchset is rebased on top of the latest vhost tree.
>>>>
>>>> [1] Reducing vdpa migration downtime because of memory pin / maps
>>>> https://www.mail-archive.com/qemu-devel@nongnu.org/msg953755.html
>>>>
>>>> ---
>>>> v4:
>>>> - Rework compatibility using new .compat_reset driver op
>>> I still think having a set_backend_feature()
>> This will overload backend features with the role of carrying over
>> compatibility quirks, which I tried to avoid from. While I think the
>> .compat_reset from the v4 code just works with the backend features
>> acknowledgement (and maybe others as well) to determine, but not
>> directly tie it to backend features itself. These two have different
>> implications in terms of requirement, scope and maintaining/deprecation,
>> better to cope with compat quirks in explicit and driver visible way.
>>
>>>    or reset_map(clean=true) might be better.
>> An explicit op might be marginally better in driver writer's point of
>> view. Compliant driver doesn't have to bother asserting clean_map never
>> be true so their code would never bother dealing with this case, as
>> explained in the commit log for patch 5 "vhost-vdpa: clean iotlb map
>> during reset for older userspace":
>>
>> "
>>       The separation of .compat_reset from the regular .reset allows
>>       vhost-vdpa able to know which driver had broken behavior before, so it
>>       can apply the corresponding compatibility quirk to the individual
>> driver
>>       whenever needed.  Compared to overloading the existing .reset with
>>       flags, .compat_reset won't cause any extra burden to the implementation
>>       of every compliant driver.
>> "
>>
>>>    As it tries hard to not introduce new stuff on the bus.
>> Honestly I don't see substantial difference between these other than the
>> color. There's no single best solution that stands out among the 3. And
>> I assume you already noticed it from all the above 3 approaches will
>> have to go with backend features negotiation, that the 1st vdpa reset
>> before backend feature negotiation will use the compliant version of
>> .reset that doesn't clean up the map. While I don't think this nuance
>> matters much to existing older userspace apps, as the maps should
>> already get cleaned by previous process in vhost_vdpa_cleanup(), but if
>> bug-for-bug behavioral compatibility is what you want, module parameter
>> will be the single best answer.
>>
>> Regards,
>> -Siwei
>>
>>> But we can listen to others for sure.
>>>
>>> Thanks
>>>