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

[Zhao Liu <zhao1.liu@intel.com>]

On Mon, Jun 03, 2024 at 07:31:47PM +0200, Philippe Mathieu-Daudé wrote:
> Date: Mon, 3 Jun 2024 19:31:47 +0200
> From: Philippe Mathieu-Daudé <philmd@linaro.org>
> Subject: Re: [PATCH-for-9.0?] docs: i386: pc: Update maximum CPU numbers
>  for PC Q35
> 
> 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

For tcg, it needs to set x2apic=on in -cpu.