Re: [RFC v3 8/8] virtio-blk: use BDRV_REQ_REGISTERED_BUF optimization hint

[Stefan Hajnoczi]

On Thu, Jul 14, 2022 at 12:16:16PM +0200, Hanna Reitz wrote:
> On 08.07.22 06:17, 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.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >   include/hw/virtio/virtio-blk.h |  2 ++
> >   hw/block/virtio-blk.c          | 13 +++++++++----
> >   2 files changed, 11 insertions(+), 4 deletions(-)
> 
> Seems fair, but as said on patch 5, I’m quite wary of “register guest RAM”. 
> How can we guarantee that it won’t be too fragmented to be registerable with
> either nvme.c or blkio.c?

We can't guarantee it. blkio instances have a maximum number of mappings
and we might exceed it. This patch doesn't have a smart solution.

Smart solutions are possible, but I haven't had time to work on one yet.
It is necessary to keep track of which mappings are referenced by
in-flight requests. When the maximum number of mappings is hit, a
mapping that currently has no references can be evicted to make space.
When the maximum number of mappings is reached by in-flight requests
then new requests may have to wait.

Until we hit the maximum number of mappings in the real world this
doesn't matter.

Stefan

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