Re: [Qemu-devel] E5-2620v2 - emulation stop error

[Bandan Das <bsd@redhat.com>]

"Kevin O'Connor" <kevin@koconnor.net> writes:

> On Wed, Mar 11, 2015 at 01:09:42PM -0400, Bandan Das wrote:
>> "Kevin O'Connor" <kevin@koconnor.net> writes:
>> ...
>> >
>> > Something is very odd here.  When I run the above command (on an older
>> > AMD machine) I get:
>> >
>> > Found 128 cpu(s) max supported 128 cpu(s)
>> >
>> > That first value (1 vs 128) comes from QEMU (via cmos index 0x5f).
>> > That is, during smp init, SeaBIOS expects QEMU to tell it how many
>> > cpus are active, and SeaBIOS waits until that many CPUs check in from
>> > its SIPI request before proceeding.
>> >
>> > I wonder if QEMU reported only 1 active cpu via that cmos register,
>> > but more were actually active.  If that was the case, it could
>> 
>> I was daring enough to try this and I don't see the crash :)
>> 
>> diff --git a/src/fw/smp.c b/src/fw/smp.c
>> index a466ea6..a346d46 100644
>> --- a/src/fw/smp.c
>> +++ b/src/fw/smp.c
>> @@ -49,6 +49,7 @@ int apic_id_is_present(u8 apic_id)
>>  void VISIBLE32FLAT
>>  handle_smp(void)
>>  {
>> +  dprintf(DEBUG_HDL_smp, "Calling handle_smp\n");
>>      if (!CONFIG_QEMU)
>>          return;
>>  
>> @@ -128,6 +129,8 @@ smp_setup(void)
>>  
>>      // Wait for other CPUs to process the SIPI.
>>      u8 cmos_smp_count = rtc_read(CMOS_BIOS_SMP_COUNT) + 1;
>> +    while (cmos_smp_count == 1)
>> +        cmos_smp_count = rtc_read(CMOS_BIOS_SMP_COUNT) + 1;
>
> That would loop forever if you had "-smp 1".

Sorry, I should have been more clear. What I meant is if I run
with "-smp 128" (from Dave's original reproducer), sticking this
while loop avoids the crash. So, the rtc_read eventually returns the
right number (127), as the above while loop keeps polling.

Bandan

>>      while (cmos_smp_count != CountCPUs)
>>          asm volatile(
>>              // Release lock and allow other processors to use the stack.
>> 
>> So, the while loop results in a race somehow ?
>
> No, the problem is that loop doesn't run at all, and as a result the
> other cpus end up running random code.  SeaBIOS sets up an entry
> vector for multiple cpus, wakes up the cpus, then tears down the entry
> vector.  If it tears down the entry vector before all the cpus have
> run through it, then the other cpus can end up running random code.
>
> -Kevin