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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