Re: [PATCH 4/4] vdpa/mlx5: implement .reset_map driver op

[Si-Wei Liu]

On 10/12/2023 8:04 PM, Jason Wang wrote:
> On Tue, Oct 10, 2023 at 5:05 PM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> wrote:
> > Since commit 6f5312f80183 ("vdpa/mlx5: Add support for running with
> > virtio_vdpa"), mlx5_vdpa starts with preallocate 1:1 DMA MR at device
> > creation time. This 1:1 DMA MR will be implicitly destroyed while
> > the first .set_map call is invoked, in which case callers like
> > vhost-vdpa will start to set up custom mappings. When the .reset
> > callback is invoked, the custom mappings will be cleared and the 1:1
> > DMA MR will be re-created.
> >
> > In order to reduce excessive memory mapping cost in live migration,
> > it is desirable to decouple the vhost-vdpa IOTLB abstraction from
> > the virtio device life cycle, i.e. mappings can be kept around intact
> > across virtio device reset. Leverage the .reset_map callback, which
> > is meant to destroy the regular MR on the given ASID and recreate the
> > initial DMA mapping. That way, the device .reset op can run free from
> > having to maintain and clean up memory mappings by itself.
> >
> > The cvq mapping also needs to be cleared if is in the given ASID.
> >
> > Co-developed-by: Dragos Tatulea <dtatulea@xxxxxxxxxx>
> > Signed-off-by: Dragos Tatulea <dtatulea@xxxxxxxxxx>
> > Signed-off-by: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>
> I wonder if the simulator suffers from the exact same issue.
For vdpa-sim !use_va (map using PA and with pinning) case, yes. But I'm 
not sure the situation of the vdpa-sim(-blk) use_va case, e.g. I haven't 
checked if there's dependency on today's reset behavior (coupled), and 
if QEMU vhost-vdpa backend driver is the only userspace consumer. Maybe 
Stefano knows?

I can give it a try on simulator fix but don't count me on the 
vdpa-sim(-blk) use_va part.

Regards,
-Siwei



>   If yes,
> let's fix the simulator as well?
>
> Thanks