Re: [RFC v4 11/11] virtio-blk: use BDRV_REQ_REGISTERED_BUF optimization hint

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Aug 23, 2022 at 10:01:59AM +0200, David Hildenbrand wrote:
> On 23.08.22 00:24, Stefan Hajnoczi wrote:
> > Register guest RAM using BlockRAMRegistrar and set the
> > BDRV_REQ_REGISTERED_BUF flag so block drivers can optimize memory
> > accesses in I/O requests.
> > 
> > This is for vdpa-blk, vhost-user-blk, and other I/O interfaces that rely
> > on DMA mapping/unmapping.
> 
> Can you explain why we're monitoring RAMRegistrar to hook into "guest
> RAM" and not go the usual path of the MemoryListener?

The requirements are similar to VFIO, which uses RAMBlockNotifier. We
need to learn about all guest RAM because that's where I/O buffers are
located.

Do you think RAMBlockNotifier should be avoided?

> What will BDRV_REQ_REGISTERED_BUF actually do? Pin all guest memory in
> the worst case such as io_uring fixed buffers would do ( I hope not ).

BLK_REQ_REGISTERED_BUF is a hint that no bounce buffer is necessary
because the I/O buffer is located in memory that was previously
registered with bdrv_registered_buf().

The RAMBlockNotifier calls bdrv_register_buf() to let the libblkio
driver know about RAM. Some libblkio drivers ignore this hint, io_uring
may use the fixed buffers feature, vhost-user sends the shared memory
file descriptors to the vhost device server, and VFIO/vhost may pin
pages.

So the blkio block driver doesn't add anything new, it's the union of
VFIO/vhost/vhost-user/etc memory requirements.

Stefan

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