Re: [RFC v3 5/8] block: add BlockRAMRegistrar

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Jul 14, 2022 at 11:30:11AM +0200, Hanna Reitz wrote:
> On 08.07.22 06:17, Stefan Hajnoczi wrote:
> > Emulated devices and other BlockBackend users wishing to take advantage
> > of blk_register_buf() all have the same repetitive job: register
> > RAMBlocks with the BlockBackend using RAMBlockNotifier.
> > 
> > Add a BlockRAMRegistrar API to do this. A later commit will use this
> > from hw/block/virtio-blk.c.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >   MAINTAINERS                          |  1 +
> >   include/sysemu/block-ram-registrar.h | 30 +++++++++++++++++++++
> >   block/block-ram-registrar.c          | 39 ++++++++++++++++++++++++++++
> >   block/meson.build                    |  1 +
> >   4 files changed, 71 insertions(+)
> >   create mode 100644 include/sysemu/block-ram-registrar.h
> >   create mode 100644 block/block-ram-registrar.c
> 
> What memory is handled in ram_list?  Is it everything?  If so, won’t devices
> have trouble registering all those buffer, especially if they happen to be
> fragmented in physical memory? (nvme_register_buf() seems to say it can run
> out of slots quite easily.)

I replied to this in another sub-thread. You are right, there is a
possibility of running out of mappings and there's no smart resource
management at the moment.

> 
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index 50f340d9ee..d16189449f 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -2490,6 +2490,7 @@ F: block*
> >   F: block/
> >   F: hw/block/
> >   F: include/block/
> > +F: include/sysemu/block-*.h
> >   F: qemu-img*
> >   F: docs/tools/qemu-img.rst
> >   F: qemu-io*
> 
> Sneaky. ;)
> 
> > diff --git a/include/sysemu/block-ram-registrar.h b/include/sysemu/block-ram-registrar.h
> > new file mode 100644
> > index 0000000000..09d63f64b2
> > --- /dev/null
> > +++ b/include/sysemu/block-ram-registrar.h
> > @@ -0,0 +1,30 @@
> > +/*
> > + * BlockBackend RAM Registrar
> > + *
> > + * SPDX-License-Identifier: GPL-2.0-or-later
> > + */
> > +
> > +#ifndef BLOCK_RAM_REGISTRAR_H
> > +#define BLOCK_RAM_REGISTRAR_H
> > +
> > +#include "exec/ramlist.h"
> > +
> > +/**
> > + * struct BlockRAMRegistrar:
> > + *
> > + * Keeps RAMBlock memory registered with a BlockBackend using
> > + * blk_register_buf() including hotplugged memory.
> > + *
> > + * Emulated devices or other BlockBackend users initialize a BlockRAMRegistrar
> > + * with blk_ram_registrar_init() before submitting I/O requests with the
> > + * BLK_REQ_REGISTERED_BUF flag set.
> 
> s/BLK/BDRV/, right?

Thanks, fixed!

Stefan

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