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

Avi Kivity
2009-06-16T08:53:42Z

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.

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error compiling committee.c: too many arguments to function