linux-kernel - Re: [PATCH] KVM: Handle MSR_IA32_PERF

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Message-ID: <20160527152224.GA11721@potion>
Date:	Fri, 27 May 2016 17:22:25 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	kmeaw@...dex-team.ru
Cc:	"Gabriel L. Somlo" <gsomlo@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"gleb@...nel.org" <gleb@...nel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: Handle MSR_IA32_PERF_CTL

2016-05-27 09:49+0300, kmeaw@...dex-team.ru:
> 26.05.2016, 23:44, "Gabriel L. Somlo" <gsomlo@...il.com>:
>> On Thu, May 26, 2016 at 10:39:31PM +0200, Radim Krčmář wrote:
>>>  2016-05-26 10:32+0300, kmeaw@...dex-team.ru:
>>>  > This patch implements a dummy handler for MSR_IA32_PERF_CTL to avoid the
>>>  > crashes. Most notably it fixes an issue with MacOS X 10.10 kernel.
>>>  Does MacOS X write it too?
>>
>> After setting /sys/module/kvm/parameters/ignore_msrs, all I get in
>> dmesg after firing up OS X is:
>>
>>         vcpu0 ignored rdmsr: 0x199
>>
>> So no, I don't think it would try to write it.
> 
> That's right, OS X does not issue an wrmsr to 0x199. More specifically, I have not
> observed that on my KVM instances. Should I remove the "wrmsr" portion from the
> patch?

Yes, please.  Silently ignoring the write is worse than #GP and #GP is
not a problem, so I wouldn't bother with a phony implementation.
Returning 0 on read is ok as seems to mean P-state=0, which is within
expectations.

(I wonder why MacOS X doesn't read IA32_PERF_STATUS, though.)