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

[Lei Yang]

QE tested this series v4 with regression testing on real nic, there is
no new regression bug.

Tested-by: Lei Yang <leiyang@xxxxxxxxxx>

On Tue, Oct 24, 2023 at 6:02 AM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx/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
> >
>