Re: [PATCH v2 1/5] hw: Add sockets_specified field in CpuTopology

[Daniel P . Berrangé]

On Wed, May 27, 2020 at 10:01:21AM +0000, Anup Patel wrote:
> 
> 
> > -----Original Message-----
> > From: Daniel P. Berrangé <address@hidden>
> > Sent: 27 May 2020 15:21
> > To: Anup Patel <address@hidden>
> > Cc: Eduardo Habkost <address@hidden>; Marcel Apfelbaum
> > <address@hidden>; Peter Maydell <address@hidden>;
> > Palmer Dabbelt <address@hidden>; Alistair Francis
> > <address@hidden>; Sagar Karandikar <address@hidden>;
> > Atish Patra <address@hidden>; address@hidden; qemu-
> > address@hidden; Anup Patel <address@hidden>
> > Subject: Re: [PATCH v2 1/5] hw: Add sockets_specified field in CpuTopology
> > 
> > On Wed, May 27, 2020 at 09:48:39AM +0000, Anup Patel wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: Daniel P. Berrangé <address@hidden>
> > > > Sent: 27 May 2020 14:16
> > > > To: Anup Patel <address@hidden>
> > > > Cc: Eduardo Habkost <address@hidden>; Marcel Apfelbaum
> > > > <address@hidden>; Peter Maydell
> > > > <address@hidden>; Palmer Dabbelt <address@hidden>;
> > > > Alistair Francis <address@hidden>; Sagar Karandikar
> > > > <address@hidden>; Atish Patra <address@hidden>;
> > > > address@hidden; qemu- address@hidden; Anup Patel
> > > > <address@hidden>
> > > > Subject: Re: [PATCH v2 1/5] hw: Add sockets_specified field in
> > > > CpuTopology
> > > >
> > > > On Wed, May 27, 2020 at 11:12:22AM +0530, Anup Patel wrote:
> > > > > When "sockets" sub-option of "-smp" option is not specified, the
> > > > > smp_parse() function will assume one CPU per-socket and set the
> > > > > number of sockets equal to number of CPUs.
> > > > >
> > > > > This is counter-intuitive and we should allow machine emulation to
> > > > > decide default number of sockets when "sockets" sub-option is not
> > > > > specified.
> > > >
> > > > I don't agree with this.  Having the semantics of the -smp option be
> > > > the same across all targets/machines *is* intuitive.  Changing
> > > > semantics of -smp per- machine will create a worse experiance for
> > > > people configuring QEMU as the configuration will mean different things
> > depending on the machine choce.
> > >
> > > Okay then why don't we default to sockets=1 in smp_parse() when "sockets"
> > > sub-options is not specified ?? This will make it uniform across machines.
> > >
> > > Is there a reason to by default have sockets=max_cpus ??
> > 
> > IIUC both of these questions are due to backwards compatibility with pre-
> > existing QEMU versions.
> 
> I see that hw/x86/pc.c implements it's own smp_parse() callback.
> Can we take that route ??
> 
> We need to have sockets=1 by default for RISC-V machines because
> each socket has it's own interrupt controller and other peripherals.

I guess the fact that smp_parse() exists as a callback pretty much
says that allowing machine type overrides of default semantics is
permitted. So yeah, using a smp_parse callback seems reasonable.

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|