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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