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

[Ingo Molnar]

* Avi Kivity <avi@xxxxxxxxxx> wrote:

> 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).

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?

	Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/