Re: [Xen-devel] [PATCH] x86/hvm: add support for MSR 0x35

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.09.14 at 06:08, <eshelton@xxxxxxxxx> wrote:
> OS X interrogates MSR 0x35 to determine the number of enabled cores and
> threads, and will fail to boot if reasonable values are not returned.
> According to the OS X kernel source code (see osfmk/i386/cpuid.c), on
> Westmere bits 19:16 are the core count and bits 15:0 are the thread
> count, and on later families bits 31:16 are the core count and bits 15:0
> are the thread count.  VirtualBox returns similar values (see
> CPUMAllRegs.cpp).

First of all - the title saying "MSR 0x35" is pretty meaningless.

This is even more so that the MSR here doesn't appear to be
documented. It's not only not an architectural MSR (i.e. you
misnamed it in msr-index.h), I can't find _any_ mention of it in
Intel's SDM. Hence a minimal requirement for accepting a patch
like this is that you point us to existing (public) documentation.
The OS X kernel source is clearly not an acceptable source here.

And then, this being - presumably, according to your description
above - an Intel-specific MSR, why would we want to also emulate
it on SVM?

Plus finally - isn't the information available through CPUID leaves?
If so, why would OS X not use this architectural way of getting
hold of the desired information?

Jan


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