Re: [PATCH] cpu hotplug issue

Gleb Natapov
2011-08-03T10:38:47Z

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.