Re: [Qemu-devel] Meaning of "-smp threads" on mips_malta

[Eduardo Habkost]

On Tue, Jan 15, 2019 at 03:28:50AM +0100, Aleksandar Markovic wrote:
> On Monday, January 14, 2019, Eduardo Habkost <address@hidden> wrote:
> 
> > Hi,
> >
> > I'm trying to refactor the SMP topology code in QEMU
> 
> 
> >
> Eduardo, I truly appreciate your interest in details of Malta
> implementations, but before I answer your questions, could you please
> outline the motivation and the current concept of your envisioned
> refactoring of SMP, so that I have some picture of the whole context?

I don't have any concrete plans right now.  I'm trying to
understand how existing code uses the -smp options, before
deciding what to do.

My goal is to make features like
  "[PATCH v1 0/5] Introduce cpu die topology and enable CPUID.1F for i386"
easier, less intrusive, and less risky.

My current impression is that things will be better if we move
the complex PC-specific topology rules from vl.c to PC-specific
code.  Most of the other architectures seem to have different
assumptions and expectations about -smp/cores/threads.


> 
> Sincerely, Aleksandar
> 
> 
> 
> >
> > and I found
> > some suspicious code on mips_malta.c:
> >
> > static void malta_mips_config(MIPSCPU *cpu)
> > {
> >     CPUMIPSState *env = &cpu->env;
> >     CPUState *cs = CPU(cpu);
> >
> >     env->mvp->CP0_MVPConf0 |= ((smp_cpus - 1) << CP0MVPC0_PVPE) |
> >                          ((smp_cpus * cs->nr_threads - 1) << CP0MVPC0_PTC);
> > }
> >
> >
> > The (smp_cpus * cs->nr_threads) expression here doesn't make
> > sense to me (because smp_cpus is already supposed to be a
> > multiple of smp_threads), and seems to indicate that the code has
> > some unusual assumptions about the semantics of the -smp option.
> >
> > So, I'd like to know: do all the examples below make sense for
> > Malta?
> >
> >  -smp 1
> >  -smp 2
> >  -smp 2,threads=1
> >  -smp 2,threads=2
> >  -smp 1,threads=2 [*]
> >  -smp 2,threads=3 [*]
> >
> > The generic -smp parsing code considers the last 2 entries
> > above[*] to be invalid.  If they make sense for Malta, we need to
> > find a way to fix that.  Replacing "-smp threads=..." with a
> > "-cpu" or "-machine" option seems like the best alternative.
> >
> > --
> > Eduardo
> >
> >

-- 
Eduardo