Re: 2.6.27.5 guest boot failure using in-kernel PIT

[Jan Kiszka <jan.kiszka@web.de>]

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Eduardo Habkost wrote:
> On Fri, Nov 21, 2008 at 08:54:56AM +0100, Jan Kiszka wrote:
>> Eduardo Habkost wrote:
>>> On Thu, Nov 20, 2008 at 12:22:53PM -0200, Eduardo Habkost wrote:
>>>> Hi,
>>>>
>>>> When using a kvm.git kernel as host, I am getting guest boot failures
>>>> when booting Fedora Rawhide kernel (2.6.27.5-117.fc10.x86_64). Guest
>>>> stops booting at:
>>>>
>>>> ENABLING IO-APIC IRQs
>>>> ..TIMER: vector=0x30 apic1=0 pin1=0 apic2=-1 pin2=-1
>>>> ..MP-BIOS bug: 8254 timer not connected to IO-APIC
>>>> ...trying to set up timer (IRQ0) through the 8259A ...
>>>> ..... (found apic 0 pin 0) ...
>>>> ....... failed.
>>>> ...trying to set up timer as Virtual Wire IRQ...
>>>> ..... failed.
>>>> ...trying to set up timer as ExtINT IRQ...
>>> I've just found out this problem happens because the guest has HZ=1000
>>> and the host had HZ=250 and no CONFIG_HIGH_RES_TIMERS.
>>>
>>> With this setup, the host is not managing to inject enough timer
>>> interrupts during the mdelay() loop on timer_irq_works().
>>>
>> Interesting, and plausible.
>>
>> My observation so far is a sporadic test failure, often correlating with
>> some raised host OS load. I'm running a high-res kernel, but that cannot
>> prevent that this only 10 ticks long loop of the guest may obtain too
>> few CPU cycles to handle enough of them once in a while (IIRC, it needs
>> 4 out of the 10 ticks to declare the timer routing functional).
>>
>> Maybe Gleb's anti-coalesce patches for the PIC can also deal with your
>> timer resolution conflict. At least worth a try...
> 
> Aren't Gleb patches for the userspace PIT?

Yeah, they are, so you won't benefit from them for in-kernel cases. But
with in-kernel emulation just the probability of coalesced ticks is a
bit lower, they cannot be avoided either.

> I am seeing the problem here
> when using the in-kernel PIT, but (surprisingly) my setup works when
> using -no-kvm-pit.

Weird, makes no sense to me as well ATM.

Jan


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