linux-kernel - Re: [RFC v2 PATCH] vdpa_sim: implement .reset

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Message-ID: <tqjrjw64l7pqphaagskys4zmkknkv46w7fxojic75b5dwbtq7m@dgq3xgryqz6f>
Date:   Thu, 19 Oct 2023 11:29:18 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Si-Wei Liu <si-wei.liu@...cle.com>
Cc:     jasowang@...hat.com, mst@...hat.com, eperezma@...hat.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 PATCH] vdpa_sim: implement .reset_map support

On Wed, Oct 18, 2023 at 04:47:48PM -0700, Si-Wei Liu wrote:
>
>
>On 10/18/2023 1:05 AM, Stefano Garzarella wrote:
>>On Tue, Oct 17, 2023 at 10:11:33PM -0700, Si-Wei Liu wrote:
>>>RFC only. Not tested on vdpa-sim-blk with user virtual address.
>>>Works fine with vdpa-sim-net which uses physical address to map.
>>>
>>>This patch is based on top of [1].
>>>
>>>[1] https://lore.kernel.org/virtualization/1696928580-7520-1-git-send-email-si-wei.liu@oracle.com/
>>>
>>>Signed-off-by: Si-Wei Liu <si-wei.liu@...cle.com>
>>>
>>>---
>>>RFC v2:
>>> - initialize iotlb to passthrough mode in device add
>>
>>I tested this version and I didn't see any issue ;-)
>Great, thank you so much for your help on testing my patch, Stefano!

You're welcome :-)

>Just for my own interest/curiosity, currently there's no vhost-vdpa 
>backend client implemented for vdpa-sim-blk

Yep, we developed libblkio [1]. libblkio exposes common API to access 
block devices in userspace. It supports several drivers.
The one useful for this use case is `virtio-blk-vhost-vdpa`. Here [2] 
some examples on how to use the libblkio test suite with the 
vdpa-sim-blk.

Since QEMU 7.2, it supports libblkio drivers, so you can use the 
following options to attach a vdpa-blk device to a VM:

   -blockdev node-name=drive_src1,driver=virtio-blk-vhost-vdpa,path=/dev/vhost-vdpa-0,cache.direct=on \
   -device virtio-blk-pci,id=src1,bootindex=2,drive=drive_src1 \

For now only what we called slow-path [3][4] is supported, since the VQs 
are not directly exposed to the guest, but QEMU allocates other VQs 
(similar to shadow VQs for net) to support live-migration and QEMU 
storage features. Fast-path is on the agenda, but on pause for now.

>or any vdpa block device in userspace as yet, correct? 

Do you mean with VDUSE?
In this case, yes, qemu-storage-daemon supports it, and can implement a 
virtio-blk in user space, exposing a disk image thorough VDUSE.

There is an example in libblkio as well [5] on how to start it.

>So there was no test specific to vhost-vdpa that needs to be exercised, 
>right?
>

I hope I answered above :-)
This reminded me that I need to write a blog post with all this 
information, I hope to do that soon!

Stefano

[1] https://gitlab.com/libblkio/libblkio
[2] https://gitlab.com/libblkio/libblkio/-/blob/main/tests/meson.build?ref_type=heads#L42
[3] https://kvmforum2022.sched.com/event/15jK5/qemu-storage-daemon-and-libblkio-exploring-new-shores-for-the-qemu-block-layer-kevin-wolf-stefano-garzarella-red-hat
[4] https://kvmforum2021.sched.com/event/ke3a/vdpa-blk-unified-hardware-and-software-offload-for-virtio-blk-stefano-garzarella-red-hat
[5] https://gitlab.com/libblkio/libblkio/-/blob/main/tests/meson.build?ref_type=heads#L58