Re: [RFC v2 PATCH] vdpa_sim: implement .reset_map support

[Si-Wei Liu]

On 10/19/2023 2:29 AM, Stefano Garzarella wrote:
> 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@xxxxxxxxxx/
> > > >
> > > > Signed-off-by: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>
> > > >
> > > > ---
> > > > 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 :-)
Definitely! This is exactly what I needed, it's really useful! Much 
appreciated for the detailed information!

I hadn't been aware of the latest status on libblkio drivers and qemu 
support since I last checked it (it was at some point right after KVM 
2022, sorry my knowledge too outdated). I followed your links below and 
checked a few things, looks my change shouldn't affect anything. Good to 
see all the desired pieces landed to QEMU and libblkio already as 
planned, great job done!

Cheers,
-Siwei

> 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