Re: [PATCH] Write to read-only msr MSR_IA32_PERF_STATUS is harmless, ignore it!

[Jes Sorensen <Jes.Sorensen@redhat.com>]

On 08/31/10 18:44, Avi Kivity wrote:
>  On 08/31/2010 07:33 PM, Jes Sorensen wrote:
>> On 08/31/10 18:28, Avi Kivity wrote:
>>>   On 08/31/2010 03:17 PM, Jes.Sorensen@redhat.com wrote:
>>>> From: Jes Sorensen<Jes.Sorensen@redhat.com>
>>>>
>>>> We regularly see bug reports over this one, however it is a write to
>>>> a read-only register which some operating systems (including Linux)
>>>> tend to write to once in a while.
>>>>
>>>> Ignore the writes since they do no harm.
>>>>
>>> Does Linux write it with wrmsr_safe()?  If not, I don't see how it
>>> works.  If it does, then we shouldn't ignore the write, instead issue
>>> the #GP as usual, but be silent about it.
>> To be honest, I am having a hard time determining where the write
>> happens. I looked at this with Gleb and the rip obtained by
>> kvm_read_rip(vcpu) in the code doesn't indicate anything that resembles
>> a wrmsr().
>>
>> It was either a
>> 560                    outb(0xfe, 0x64); /* pulse reset low */
>> or
>> 49        asm volatile("sti; hlt": : :"memory");
>>
>> which makes no sense to me,
> 
> Just grep for the msr name in a guest kernel source that's known to
> trigger the message.

Been there, done that! This happens with an F13 kernel during reboot.
Ran the search on the expanded 2.6.32.8-149 tree and found no reference
to anything trying to write it, except for KVM backing up the flag, but
that shouldn't happen in the guest.

>>   but given it's x86, I am not sure if it
>> could have come from the BIOS or something during reboot?
> 
> The bios is the same for all kernels (and is unlikely to mess with
> performance counter msrs anyway).

I was fooled by this too, it's not a performance counter MSR, it's a CPU
frequency scaling MSR.

Cheers,
Jes