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

[Avi Kivity]

On 06/15/2009 09:55 PM, H. Peter Anvin wrote:
> Ingo Molnar wrote:
>    
> > > I wouldn't actually expect that, *as long as* there is
> > > serialization between the cr2 write and the cr2 read.
> > >        
> > Well, is there any OS that heavily relies on cr2 writes and which
> > uses them from NMI context, and which CPU makers care about?
> > (Meaning: Windows, pretty much.)
> >
> > If not then i agree that in theory it should work fine, but in
> > practice we only know that we dont know the unknown risk here ;-)
> >
> >      
>
> I think you can drop "uses them from NMI context" from that statement;
> writing to %cr2 is independent of the context.
>
> 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).


--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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