Hi, OpenBSD/i386 seems to be one of the few operating systems that still uses the LAPIC taks priority register for interrupt handling. On AMD CPUs and on older Intel CPUs without the flexpriority feature, this causes a huge performance impact on kvm. I have seen slowdown by a factor of 10. Is there a way to use the TPR under kvm without the slowdown? There are some MSRs inherited from Hyper-V, but using these does not make that much difference. I think this is because they still cause an vmexit for every TPR access. I expect the the same is true for x2apic emulation, isn't it? There is also the kvmvapic, but kvm does not expose a sane interface to it and only uses it for Windows XP specific binary patching. Another possibility is TPR access via CR8 on AMD, but the missing cr8_legacy CPUID bit and this discussion [1] make me believe that this is not supported under kvm, at least in 32bit mode. Could this be easily fixed? If yes, would it solve the performance problems, i.e. offer performance comparable to Intel's flexpriority feature? OpenBSD seems to be reluctant to stop using the TPR. In fact, in a recent discussion, there has been a suggestion that OpenBSD should switch to using TPR also on OpenBSD/amd64 to solve some problems with boot interrupts. How do you expect this would affect performance under kvm (if using CR8)? Or do you have any other suggestions? One could also modify kvm to expose a real interface to kvmvapic, e.g. allow the guest OS to provide the virtual address of the option rom and the offset of the CPU number in the %fs segment, instead of using hard coded values for Windows XP. Cheers, Stefan [1] http://www.mail-archive.com/kvm@xxxxxxxxxxxxxxx/msg30627.html -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
