linux-kernel - Re: [PATCH 4/4] vdpa/mlx5: implement .reset

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Message-ID: <97cd1765-a427-4981-be2f-004bd86574aa@oracle.com>
Date:   Fri, 13 Oct 2023 00:55:03 -0700
From:   Si-Wei Liu <si-wei.liu@...cle.com>
To:     Jason Wang <jasowang@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>
Cc:     mst@...hat.com, eperezma@...hat.com, xuanzhuo@...ux.alibaba.com,
        dtatulea@...dia.com, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] vdpa/mlx5: implement .reset_map driver op



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@...cle.com> 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@...dia.com>
>> Signed-off-by: Dragos Tatulea <dtatulea@...dia.com>
>> Signed-off-by: Si-Wei Liu <si-wei.liu@...cle.com>
> 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
>