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

Avi Kivity
2009-06-15T19:03:54Z

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.