Re: [PATCH-for-9.0?] docs: i386: pc: Update maximum CPU numbers for PC Q35

[Philippe Mathieu-Daudé]

Hi Michael,

On 2/6/24 15:30, Michael S. Tsirkin wrote:
> On Fri, Apr 12, 2024 at 11:57:35AM +0200, Philippe Mathieu-Daudé wrote:
> > Hi Zhao,
> >
> > On 12/4/24 10:53, Zhao Liu wrote:
> > > From: Zhao Liu <zhao1.liu@intel.com>
> > >
> > > Commit e4e98c7eebfa ("pc: q35: Bump max_cpus to 4096 vcpus") increases
> > > the supported CPUs for PC Q35 machine.
> > >
> > > Update maximum CPU numbers for PC Q35 in the document.
> > >
> > > Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> > > ---
> > >    docs/system/target-i386-desc.rst.inc | 2 +-
> > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/docs/system/target-i386-desc.rst.inc 
> > > b/docs/system/target-i386-desc.rst.inc
> > > index 5ebbcda9db4c..319e540573d3 100644
> > > --- a/docs/system/target-i386-desc.rst.inc
> > > +++ b/docs/system/target-i386-desc.rst.inc
> > > @@ -36,7 +36,7 @@ The QEMU PC System emulator simulates the following 
> > > peripherals:
> > >    -  PCI UHCI, OHCI, EHCI or XHCI USB controller and a virtual USB-1.1
> > >       hub.
> > > -SMP is supported with up to 255 CPUs.
> > > +SMP is supported with up to 255 CPUs (and 4096 CPUs for PC Q35 machine).
> >
> > This comment is not accurate since a while, IIUC:
> >
> > Up to q35-2.7: 255
> > q35-2.8: 288
> > q35-8.0+: 1024
> > q35-9.0: 4096
>
>
> What are you saying here, Philippe? I don't think compat
> machine types matter enough to bother with more detail.

My point is I find this description confusing w.r.t. how QEMU behaves:

$ qemu-system-i386 -M pc-q35-2.8 -smp 666
qemu-system-i386: Invalid SMP CPUs 666. The max CPUs supported by 
machine 'pc-q35-2.8' is 288

$ qemu-system-i386 -M pc-q35-8.0 -smp 666
qemu-system-i386: Invalid SMP CPUs 666. The max CPUs supported by 
machine 'pc-q35-8.0' is 288

$ qemu-system-i386 -M pc-q35-9.0 -smp 666
Unexpected error in apic_common_set_id() at ../hw/intc/apic_common.c:447:
qemu-system-i386: APIC ID 255 requires x2APIC feature in CPU
Abort trap: 6

I'd rather at least:

"SMP is supported with up to 255 CPUs (and since v9.0: 4096 CPUs for PC 
Q35 machine)."

or:

"SMP is supported with up to 255 CPUs (and 4096 CPUs for PC Q35 machine 
since v9.0)."

> > >    QEMU uses the PC BIOS from the Seabios project and the Plex86/Bochs LGPL
> > >    VGA BIOS.