Re: [PATCH v2 1/4] softmmu: Support concurrent bounce buffers

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Aug 23, 2023 at 04:54:06PM -0400, Peter Xu wrote:
> On Wed, Aug 23, 2023 at 10:08:08PM +0200, Mattias Nissler wrote:
> > On Wed, Aug 23, 2023 at 7:35 PM Peter Xu <peterx@redhat.com> wrote:
> > > On Wed, Aug 23, 2023 at 02:29:02AM -0700, Mattias Nissler wrote:
> > > > diff --git a/softmmu/vl.c b/softmmu/vl.c
> > > > index b0b96f67fa..dbe52f5ea1 100644
> > > > --- a/softmmu/vl.c
> > > > +++ b/softmmu/vl.c
> > > > @@ -3469,6 +3469,12 @@ void qemu_init(int argc, char **argv)
> > > >                  exit(1);
> > > >  #endif
> > > >                  break;
> > > > +            case QEMU_OPTION_max_bounce_buffer_size:
> > > > +                if (qemu_strtosz(optarg, NULL, &max_bounce_buffer_size) < 0) {
> > > > +                    error_report("invalid -max-ounce-buffer-size value");
> > > > +                    exit(1);
> > > > +                }
> > > > +                break;
> > >
> > > PS: I had a vague memory that we do not recommend adding more qemu cmdline
> > > options, but I don't know enough on the plan to say anything real.
> > 
> > I am aware of that, and I'm really not happy with the command line
> > option myself. Consider the command line flag a straw man I put in to
> > see whether any reviewers have better ideas :)
> > 
> > More seriously, I actually did look around to see whether I can add
> > the parameter to one of the existing option groupings somewhere, but
> > neither do I have a suitable QOM object that I can attach the
> > parameter to, nor did I find any global option groups that fits: this
> > is not really memory configuration, and it's not really CPU
> > configuration, it's more related to shared device model
> > infrastructure... If you have a good idea for a home for this, I'm all
> > ears.
> 
> No good & simple suggestion here, sorry.  We can keep the option there
> until someone jumps in, then the better alternative could also come along.
> 
> After all I expect if we can choose a sensible enough default value, this
> new option shouldn't be used by anyone for real.

QEMU commits to stability in its external interfaces. Once the
command-line option is added, it needs to be supported in the future or
go through the deprecation process. I think we should agree on the
command-line option now.

Two ways to avoid the issue:
1. Drop the command-line option until someone needs it.
2. Make it an experimental option (with an "x-" prefix).

The closest to a proper solution that I found was adding it as a
-machine property. What bothers me is that if QEMU supports
multi-machine emulation in a single process some day, then the property
doesn't make sense since it's global rather than specific to a machine.

CCing Markus Armbruster for ideas.

Stefan

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