Re: GPF in intel_pmu_lbr_reset() with qemu -cpu host

Gleb Natapov
2014-03-22T19:03:56Z

On Sat, Mar 22, 2014 at 11:05:03AM +0100, Peter Wu wrote:
> On Saturday 22 March 2014 10:50:45 Gleb Natapov wrote:
> > On Fri, Mar 21, 2014 at 12:04:32PM -0700, Venkatesh Srinivas wrote:
> > > On Fri, Mar 21, 2014 at 10:46 AM, Peter Wu <peter@lekensteyn.nl> wrote:
> > [skip]
> >
> > > When -cpu host is used, qemu/kvm passed the host CPUID F/M/S to the
> > > guest. intel_pmu_cpu_*() -> intel_pmu_lbr_reset() uses rdmsr() /
> > > wrmsr(), rather than the safe variants; if KVM does not support the
> > > particular MSRs in question, you will see a #GP(0) there. See
> > > https://lkml.org/lkml/2014/3/13/453 for a similar bug other PMU code.
> > >
> > When kernel is compiled with guest support all rdmsr()/wrmsr() become _safe(),
> > so the question for Peter is if his guest kernel has guest support enabled?
>
> Linux guest support (CONFIG_HYPERVISOR_GUEST) was not enabled, see
> .config in the first mail[1]. Enabling that option does not change the
> situation.
>
> With CONFIG_PARAVIRT and CONFIG_KVM_GUEST enabled, the PMU GPF is gone,
Yeah, it should be PARAVIRT indeed since rdmsr()/wrmsr() is substituted by _safe()
using paravirt calls.

> but now I have a NULL dereference (in rapl_pmu_init). Previously, when
> `-cpu SandyBridge` was passed to qemu, it would show this:
>
> [ 0.016995] Performance Events: unsupported p6 CPU model 42 no PMU driver, software events only.
>
> The same NULL pointer deref would be visible (slightly different
> addresses, but the Code lines are equal). With `-host`, the NULL deref
> with `-cpu host` contains:
>
> [ 0.016445] Performance Events: 16-deep LBR, IvyBridge events, Intel PMU driver.
>
> Full dmesg below.
>
I am confused. Do you see crash now with -cpu SandyBridge and -cpu host, or -cpu host only?

--
Gleb.