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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, May 27, 2020 at 10:01:21AM +0000, Anup Patel wrote:
> 
> 
> > -----Original Message-----
> > From: Daniel P. Berrangé <berrange@redhat.com>
> > Sent: 27 May 2020 15:21
> > To: Anup Patel <Anup.Patel@wdc.com>
> > Cc: Eduardo Habkost <ehabkost@redhat.com>; Marcel Apfelbaum
> > <marcel.apfelbaum@gmail.com>; Peter Maydell <peter.maydell@linaro.org>;
> > Palmer Dabbelt <palmer@dabbelt.com>; Alistair Francis
> > <Alistair.Francis@wdc.com>; Sagar Karandikar <sagark@eecs.berkeley.edu>;
> > Atish Patra <Atish.Patra@wdc.com>; qemu-riscv@nongnu.org; qemu-
> > devel@nongnu.org; Anup Patel <anup@brainfault.org>
> > 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 <berrange@redhat.com>
> > > > Sent: 27 May 2020 14:16
> > > > To: Anup Patel <Anup.Patel@wdc.com>
> > > > Cc: Eduardo Habkost <ehabkost@redhat.com>; Marcel Apfelbaum
> > > > <marcel.apfelbaum@gmail.com>; Peter Maydell
> > > > <peter.maydell@linaro.org>; Palmer Dabbelt <palmer@dabbelt.com>;
> > > > Alistair Francis <Alistair.Francis@wdc.com>; Sagar Karandikar
> > > > <sagark@eecs.berkeley.edu>; Atish Patra <Atish.Patra@wdc.com>;
> > > > qemu-riscv@nongnu.org; qemu- devel@nongnu.org; Anup Patel
> > > > <anup@brainfault.org>
> > > > 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
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