[Xen-devel] Re: [RFC, PATCH 17/24] i386 Vmi msr patch

[Zachary Amsden <zach@xxxxxxxxxx>]

Andi Kleen wrote:
> On Monday 13 March 2006 19:12, Zachary Amsden wrote:
>   
> > Fairly straightforward code motion of MSR / TSC / PMC accessors
> > to the sub-arch level.  Note that rdmsr/wrmsr_safe functions are
> > not moved; Linux relies on the fault behavior here in the event
> > that certain MSRs are not supported on hardware, and combining
> > this with a VMI wrapper is overly complicated.  The instructions
> > are virtualizable with trap and emulate, not on critical code
> > paths, and only used as part of the MSR /proc device, which is
> > highly sketchy to use inside a virtual machine, but must be
> > allowed as part of the compile, since it is useful on native.
> >     
> I'm not aware of any MSR access being on a critical code
> path on a 32bit kernel. 
>   
There aren't really any.  There are some unexpected ones - such as 
setting SYSENTER_CS during a context switch, but only if leaving v8086 
mode, which isn't a common case.  But most importantly, the MSR 
functions were challenging to get correct, because they combine two 
novel elements - 64 bit values, as well as non-C calling conventions.  
They were actually some of the first functions I inlined, because I knew 
there would be problems, and the solutions would yield more powerful 
inlining macros.
> And I don't think it's a good idea to virtualize the TSC 
> without CPU support.
>   
We currently don't support configurations without a TSC.  But we're not 
trying to virtualize the TSC without CPU support.  It is possible.  But 
I have no idea _why_ you would want to do such a thing.
Zach

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