Re: [PATCH 1/5] Skip timer works.patch

[Zachary Amsden <zach@vmware.com>]

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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