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

* Avi Kivity <avi@redhat.com> 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


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