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SubjectRe: perf hw in kexeced kernel broken in tip
On Wed, Dec 08, 2010 at 03:59:16PM +0100, Peter Zijlstra wrote:
> On Wed, 2010-12-08 at 15:20 +0100, Peter Zijlstra wrote:
>
> > > I wonder if you should reverse these checks. If the bios has the perf
> > > counter enabled, there might be a high chance that it fails the first
> > > check and never gets to the actually bios checks.
> >
> > Ah, good point.
>
> Something like so..

Getting closer...

Pentium4s are special they need the double write, so...

> + /*
> + * Now write a value and read it back to see if it matches,
> + * this is needed to detect certain hardware emulators (qemu/kvm)
> + * that don't trap on the MSR access and always return 0s.
> + */
> val = 0xabcdUL;
> - ret |= checking_wrmsrl(x86_pmu.perfctr, val);
> + ret = checking_wrmsrl(x86_pmu.perfctr, val);

if (x86_pmu.perfctr_second_write)
ret |= checking_wrmsrl(x86_pmu.perfctr, val);

solved my p4 problems for kexec.

> ret |= rdmsrl_safe(x86_pmu.perfctr, &val_new);
> if (ret || val != val_new)
> - return false;
> + goto msr_fail;
>
> return true;

Cheers,
Don


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