Re: [Qemu-devel] [PATCH v2 4/4] replay: introduce block devices record/replay

[Stefan Hajnoczi]

On Thu, Feb 11, 2016 at 04:52:42PM +0300, Pavel Dovgalyuk wrote:
> > From: Stefan Hajnoczi [mailto:address@hidden
> > On Wed, Feb 10, 2016 at 12:13:23PM +0300, Pavel Dovgalyuk wrote:
> > > @@ -784,7 +798,11 @@ BlockAIOCB *blk_aio_flush(BlockBackend *blk,
> > >          return blk_abort_aio_request(blk, cb, opaque, -ENOMEDIUM);
> > >      }
> > >
> > > -    return bdrv_aio_flush(blk->bs, cb, opaque);
> > > +    if (replay_mode == REPLAY_MODE_NONE) {
> > > +        return bdrv_aio_flush(blk->bs, cb, opaque);
> > > +    } else {
> > > +        return replay_aio_flush(blk->bs, cb, opaque);
> > > +    }
> > >  }
> > 
> > I am only looking at this patch in isolation and am not familiar with
> > the record/replay work, but these changes appear invasive.  In order for
> > record/replay to keep working in the future, everyone touching block
> > layer code must be familiar with the principles of how record/replay
> > works.  This patch does not include documentation.
> 
> We've already discussed it with Kevin.
> He proposes adding new block driver for record/replay.
> 
> > Can you point me to documentation that explains the how record replay
> > works?
> 
> There is some information about it in docs/replay.txt and in
> http://wiki.qemu.org/Features/record-replay

I was thinking about developer documentation.  A feature like this is
"cross-cutting" - it affects many components.  There should be
documentation that explains the semantics so everyone modifying code can
follow the rules to keep record/replay working.

Live migration is similar in that it touches many components.  The
docs/migration.txt file explains the APIs and general idea.  It
communicates the assumptions, limitations, and requirements that live
migration imposes.  Many QEMU developers understand migration basics and
keep them in mind during code review.  This way we can prevent most (but
not all) migration breakage.

Record/replay should aim for the same level of education/awareness,
otherwise the feature will break and bitrot.

The alternative is to implement it in a way that isn't invasive.  Then
other developers don't need to understand how it works.  Maybe
implementing it in a block driver can achieve this.

Stefan

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