* Avi Kivity <avi@xxxxxxxxxx> wrote: > 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 :) Heh :) > 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. Yep, see my numbers elsewhere in this thread - the cost of a cr2 write is ~84 cycles on Nehalem. Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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