>>>> Hi, >>>> >>>> According to this: >>>> >>>> https://github.com/torvalds/linux/commit/0d1de2d901f4ba0972a3886496a44fb1d3300dbd >>>> >>>> It looks like kvm have been supporting x2apic since kernel 2.6.32, or >>>> even earlier. >>>> >>> This patch is to emulate x2apic for guest, then guest can benefit from some advantages of x2apic, like using RDMSR/WRMSR instead of mmio. >>> >>>> However, this following patch: >>>> >>>> https://github.com/torvalds/linux/commit/8d14695f9542e9e0195d6e41ddaa52c32322adf5 >>>> >>>> Also claims that adding support for x2apic. But this later patch was >>>> for kernel 3.9. >>> This patch is to support virtual x2apic mode, which can virtualizing MSR-based APIC accesses by configuring MSR bitmaps, >>> you can specify some MSR-based accesses without VM exit, the other MSR-based accesses with VM exit, >>> which belongs to APIC virtualization from certain angle. >> Thanks Haoyu, I am reading the Intel x2apic manual. >> >> http://www.intel.com/content/dam/doc/specification-update/64-architecture-x2apic-specification.pdf >> >> but I don't see the so called "virtual x2apic mode", it seems that the >> manual does not mention anything about that. Do you mean that when the >> Guest OS sets bit 10 (x2apic mode enable bit) of the it virtualized >> IA32_APIC_BASE MSR, it is entering a virtual x2apic mode? But isn't >> this the same as the aforementioned "emulate x2apic for guest"? Or, >> "emulate x2apic for guest" is the foundation of "support virtual >> x2apic mode"? >> >> -Jidong >> >Oh, I found the answer, the virtualized x2apic mode is not defined in >the x2apic manual, but is defined in the intel SDM manual, chapter 29 >- APIC virtualization and virtual interrupts. Ignore my questions >please, but thank you anyway. > Yes, virtual x2apic mode is part of APICv. Guest has no idea about it is running in a virtual machine, VMM also prevent guest from enabling/disabling virtual x2apic mode or some other virtualization configurations. >-Jidong > >>>> >>>> So I am very confused: >>>> >>>> First, what's the difference between these two patches? Or say, does >>>> kvm support x2apic since kernel 2.6.32, or since kernel 3.9? >>>> >>> kvm support x2apic since kernel 2.6.32, but not support virtual x2apic mode until kernel 3.9. >>> >>>> Second, can guest use x2apic even if the host does not? (Assuming qemu >>>>has exposed this feature to guest.) The word "use" means something >>>> like, accessing x2apic registers, setting the x2apic enable bit in >>>> IA32_APICBASE MSR (i.e. bit 10). >>>> >>> Yes, you can use x2apic even if the PCPU does not support, and benefit from it, like performance bonus from MSR accesses instead of MMIO. >>> But, if I remember correctly, if your guest does not support interrupt-remmping, you only can use physical destination mode when using x2apic, >>> please see enable_IR_x2apic(). >>> >>>> -Jidong -- 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
