Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain supportto use NMI-safe methods

[Avi Kivity]

On 06/16/2009 11:36 AM, Ingo Molnar wrote:
> > > I can try to find out internally what Intel's position on writing
> > > %cr2 is, but it'll take a while; however, KVM should be able to
> > > tell you if any random OS uses %cr2 writes (as should a static
> > > disassembly of their kernel.)
> > >        
> > Linux is one such OS.  When acting as a hypervisor it writes cr2
> > to present its guests with their expected environment (any
> > hypervisor that uses virtualization extensions will of course need
> > to do this).
> >      
>
> Ah, it does save/restore it in svm_vcpu_run. VMX can do this via its
> context structure (without explicit CR manipulations in host space),
> right?
>    

It's the other way around.  svm switches the guest cr2 in hardware 
(through svm->vmcb->save.cr2).  The code you're referring to saves and 
restores the host cr2, which is completely unnecessary.  I'm currently 
in the middle of dropping it :)

vmx has no hardware support for switching cr2, so vmx_vcpu_run() 
switches it using mov cr2.  Given that it's pretty expensive, I've 
switched it to write-if-changed, which dropped 70 cycles from the vmexit 
latency.

--
error compiling committee.c: too many arguments to function

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