Re: [PATCH] cpu hotplug issue

[Gleb Natapov <gleb@redhat.com>]

On Wed, Aug 03, 2011 at 12:37:13PM +0200, Jan Kiszka wrote:
> On 2011-08-03 12:07, Vasilis Liaskovitis wrote:
> > When rebooting after a CPU hotplug in qemu-kvm, Seabios can get stuck in smp_probe().
> > Normally cmos_smp_count is read from cmos and the smp_ap_boot_code is run on all cpus 
> > except bootstrap. The expected result is CountCPUs == cmos_smp_count + 1.  After a 
> > cpu hotplug, more than cmos_smp_count cpus are active, so we get a situation where 
> > CountCPUs > cmos_smp_count + 1 and Seabios keeps looping forever in smp_probe. In some
> > cases, the while loop exit condition is tested before CountCPUs gets larger (i.e. 
> > before smp_ap_boot_code runs on all CPUs), so the hang is not always reproducible.
> > 
> > This patch introduces a new fw_cfg variable called hotplugged_cpus that gets updated from
> > qemu-kvm hoplug code. Seabios reads this variable on each call to smp_probe() and adjusts
> > the expected number of online CPUs.
> > 
> > The qemu-kvm part of this patch is against Jan Kiszka's cpu-hotplug tree:
> > git://git.kiszka.org/qemu-kvm.git queues/cpu-hotplug
> > tested with qemu-system-x86_64.
> > 
> > An alternative to this patch would be to update the smp_cpus variable in qemu-kvm and 
> > do a "cmos update" to 0x5f from the cpu-hotplug code. Access to the rtc_state (cmos device) 
> > would be required in hw/acpi_piix4.c.  This way no change to Seabios would be required.
> > 
> 
> ...
> 
> >  src/paravirt.c |   12 ++++++++++++
> >  src/paravirt.h |    2 ++
> >  src/smp.c      |    6 ++++--
> >  3 files changed, 18 insertions(+), 2 deletions(-)
> > 
> > diff --git a/src/paravirt.c b/src/paravirt.c
> > index 9cf77de..3367609 100644
> > --- a/src/paravirt.c
> > +++ b/src/paravirt.c
> > @@ -305,6 +305,18 @@ u16 qemu_cfg_get_max_cpus(void)
> >      return cnt;
> >  }
> >  
> > +u16 qemu_cfg_get_hplug_cpus(void)
> > +{
> > +    u16 cnt;
> > +
> > +    if (!qemu_cfg_present)
> > +        return 0;
> > +
> > +    qemu_cfg_read_entry(&cnt, QEMU_CFG_HPLUG_CPUS, sizeof(cnt));
> 
> Why can't Seabios read to true number online CPUs from the PIIX4 device?
> The information is there already, no need for addition PV here.
> 
Where is it in PIIX4 device?

--
			Gleb.