2016-09-14 11:40+0200, Paolo Bonzini: > On 14/09/2016 09:58, Wanpeng Li wrote: >> From: Wanpeng Li <[email protected]> >> >> I observed that kvmvapic(to optimize flexpriority=N or AMD) is used >> to boost TPR access when testing kvm-unit-test/eventinj.flat tpr case >> on my haswell desktop (w/ flexpriority, w/o APICv). Commit (8d14695f9542 >> x86, apicv: add virtual x2apic support) disable virtual x2apic mode >> completely if w/o APICv, and the author also told me that windows guest >> can't enter into x2apic mode when he developed the APICv feature several >> years ago. However, it is not truth currently, Interrupt Remapping and >> vIOMMU is added to qemu and the developers from Intel test windows 8 can >> work in x2apic mode w/ Interrupt Remapping enabled recently. >> >> This patch enables TPR shadow for virtual x2apic mode to boost >> windows guest in x2apic mode even if w/o APICv. >> >> Can pass the kvm-unit-test. > > Ok, now I see what you meant; this actually makes sense. I don't expect > much speedup though, because Linux doesn't touch the TPR and Windows is > likely going to use the Hyper-V APIC MSRs when APICv is disabled. For > this reason I'm not sure if the patch is useful in practice.
I agree with Paolo on the use case -- what configurations benefit from this change? > To test this patch, you have to run kvm-unit-tests with Hyper-V > synthetic interrupt enabled. Did you do this? The patch is buggy. MSR bitmaps are global and we'd have a CVE if one guests used synic (=> disabled apicv) and one didn't. You'd want a new set of bitmaps and assign them in vmx_set_msr_bitmap() (or completely rewrite our management).
