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Re: [Qemu-devel] [PATCH] vl.c: Check max_cpus limit instead of smp_cpus


From: Eduardo Habkost
Subject: Re: [Qemu-devel] [PATCH] vl.c: Check max_cpus limit instead of smp_cpus
Date: Thu, 8 May 2014 12:00:43 -0300
User-agent: Mutt/1.5.21 (2010-09-15)

On Thu, May 08, 2014 at 09:26:36AM +0100, Peter Maydell wrote:
> On 7 May 2014 23:20, Andreas Färber <address@hidden> wrote:
> > Am 07.05.2014 23:48, schrieb Peter Maydell:
> >> This is confusing. What is max_cpus
> >
> > Specified via -smp, defaults to actual CPUs.
> 
> I thought that was smp_cpus ...

smp_cpus is the first argument to -smp. max_cpus is the "maxcpus" option
of -smp.

> 
> >> and why do we not
> >> just set max_cpus = machine->max_cpus ?
> >
> > For the PC, machine->max_cpus specifies the technical (APIC/ACPI/...)
> > upper limit, whereas -smp limits it to saner values.
> >
> > If you think KVM, then many machines today will have less than the
> > maximum number of CPUs and you may want to run multiple VMs and you may
> > not want their vCPUs competing too much for existing CPUs, so a limit
> > makes sense.
> 
> That would imply that we want
>   max_cpus = MIN(command_line_max_cpus, machine->max_cpus);

Please don't do this. If the maxcpus value from the command-line is not
supported, QEMU should abort instead of silently ignoring the value
provided by the user.

-- 
Eduardo



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