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SubjectRe: [PATCH 1/5] Skip timer works.patch
Andi Kleen wrote:
> On Thu, Oct 19, 2006 at 05:09:22PM -0700, Zachary Amsden wrote:
>
>> Add a way to disable the timer IRQ routing check via a boot option. The
>> VMI timer code uses this to avoid triggering the pester Mingo code, which
>> probes for some very unusual and broken motherboard routings. It fires
>> 100% of the time when using a paravirtual delay mechanism instead of
>> using a realtime delay, since there is no elapsed real time, and the 4 timer
>> IRQs have not yet been delivered.
>>
>
> You mean paravirtualized udelay will not actually wait?
>

Yes, but even putting that problem aside, the timing element here is
tricky to get right in a VM.

> This implies that you can't ever use any real timer in that kind of guest,
> right?
>

No. You can use a real timer just fine. But there is no reason ever to
use udelay to busy wait for "hardware" in a virtual machine. Drivers
which are used for real hardware may turn udelay back on selectively;
but this is another patch.


>> In addition, it is entirely possible, though improbable, that this bug
>> could surface on real hardware which picks a particularly bad time to enter
>> SMM mode, causing a long latency during one of the timer IRQs.
>>
>
> We already have a no timer check option. But:
>

Really? I didn't see one that disabled the broken motherboard detection
/ workaround code, which is what we are trying to avoid here.


>> While here, make check_timer be __init.
>>
>
> So how is this supposed to work? The hypervisor would always pass that
> option? If yes that would seem rather hackish to me. We should probably
> instead probe in some way if we have the required timer hardware.
> The paravirt kernel should know anyways it is paravirt and that it doesn't
> need to probe for flakey hardware.
>

That is what this patch is building towards, but the boot option is
"free", so why not? In the meantime, it helps non-paravirt kernels
booted in a VM.

Zach
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