Re: [Qemu-devel] [PATCH for-2.8 16/18] pc: add 'etc/boot-cpus' fw_cfg file for machine with more than 255 CPUs

[Igor Mammedov]

On Sun, 7 Aug 2016 09:02:33 +0300
"Michael S. Tsirkin" <address@hidden> wrote:

> On Fri, Aug 05, 2016 at 11:46:15AM +0200, Igor Mammedov wrote:
> > Currently firmware uses 1 byte at 0x5F offset in RTC CMOS
> > to get number of CPUs present at boot. However 1 byte is
> > not enough to handle more than 255 CPUs.  So add a new
> > fw_cfg file that would allow QEMU to tell it.
> > For compat reasons add file only for machine types that
> > support more than 255 CPUs.
> > 
> > Signed-off-by: Igor Mammedov <address@hidden>  
> 
> Do we really need this? Why differentiate between boot
> cpus and others? How about reusing the hotplug
> register, such that it looks just as if all CPUs
> were just added by hotplug?
We could hijack cpu hotplug registers or even better extend it
with new command, like GET_PRESENT_CPUS_COUNT

but then we would need to tell firmware where register
is located which would lead to just adding other fw_cfg
file with register address.
So it's not better (maybe worse) than just adding
etc/boot-cpus fw_cfg file.


> 
> > ---
> >  hw/i386/pc.c | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> > index c2cd5bd..2b5581a 100644
> > --- a/hw/i386/pc.c
> > +++ b/hw/i386/pc.c
> > @@ -1235,9 +1235,11 @@ void pc_machine_done(Notifier *notifier, void *data)
> >      PCMachineState *pcms = container_of(notifier,
> >                                          PCMachineState, machine_done);
> >      PCIBus *bus = pcms->bus;
> > +    static uint16_t boot_cpus;
> >  
> >      /* set the number of CPUs */
> > -    rtc_set_memory(pcms->rtc, 0x5f, pc_present_cpus_count(pcms) - 1);
> > +    boot_cpus = pc_present_cpus_count(pcms);
> > +    rtc_set_memory(pcms->rtc, 0x5f, boot_cpus - 1);
> >  
> >      if (bus) {
> >          int extra_hosts = 0;
> > @@ -1258,8 +1260,16 @@ void pc_machine_done(Notifier *notifier, void *data)
> >  
> >      acpi_setup();
> >      if (pcms->fw_cfg) {
> > +        MachineClass *mc = MACHINE_GET_CLASS(pcms);
> > +
> >          pc_build_smbios(pcms->fw_cfg);
> >          pc_build_feature_control_file(pcms);
> > +
> > +        if (mc->max_cpus > 255) {
> > +            boot_cpus = cpu_to_le16(boot_cpus);
> > +            fw_cfg_add_file(pcms->fw_cfg, "etc/boot-cpus", &boot_cpus,
> > +                            sizeof(boot_cpus));
> > +        }
> >      }
> >  }
> >  
> > -- 
> > 2.7.4