On 15/09/2016 03:19, Wanpeng Li wrote: > 2016-09-14 20:03 GMT+08:00 Radim Krčmář <rkrcmar@xxxxxxxxxx>: >> 2016-09-14 11:40+0200, Paolo Bonzini: >>> On 14/09/2016 09:58, Wanpeng Li wrote: >>>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> >>>> >>>> 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? > > Old windows guest w/o Hyper-V synthetic interrupt support. ... but with Hyper-V synthetic interrupt support enabled in the QEMU command line. Right? Paolo >> >>> 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). > -- 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
