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

["H. Peter Anvin" <hpa@zytor.com>]

Using _safe has it's own issues if noone checks the errors.

On March 22, 2014 5:27:59 AM PDT, Gleb Natapov <gleb@kernel.org> wrote:
>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.

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