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

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Date:	Tue, 31 May 2016 03:53:52 -0400 (EDT)
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	kmeaw@...dex-team.ru, "Gabriel L. Somlo" <gsomlo@...il.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, gleb@...nel.org
Subject: Re: [PATCH] KVM: Handle MSR_IA32_PERF_CTL

> 2016-05-27 17:22+0200, Radim Krčmář:
> > (I wonder why MacOS X doesn't read IA32_PERF_STATUS, though.)
> 
> Oh, it maybe does ... we already emulate status and return 0x1000 in its
> bottom 16 bits.  I have no idea what is that supposed to mean, but I
> think we should return 0x1000 in IA32_PERF_CTL then.

It's 1000, not 0x1000 (instead, on real hardware the value is typically a
multiple of 256).  It was added for Darwin too.

Returning different values is okay, because they are different on real
hardware too:

(sudo dd if=/dev/cpu/0/msr skip=$((0x198)) iflag=skip_bytes bs=8 count=1;
 sudo dd if=/dev/cpu/0/msr skip=$((0x199)) iflag=skip_bytes bs=8 count=1) | od -tx8
0000000 00001f3900001100 0000000000001300
        ^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^
        PERF_STATUS      PERF_CTL

And perhaps if we returned non-zero values for PERF_CTL Darwin would try to
write to it.  So returning zero is fine, I think.  There is no correct answer...

Paolo

> (Would be nice to understand how that 0x1000 happened ... we might want
>  0 in both.)
>