Re: [PATCH] cpu hotplug issue

Jan Kiszka
2011-08-03T10:42:21Z

On 2011-08-03 12:38, Gleb Natapov wrote:
> 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?

PROC registers (or however they are called).

Jan

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