On Tue, Jun 10, 2014 at 11:54 PM, Jidong Xiao <jidong.xiao@xxxxxxxxx> wrote: > On Tue, Jun 10, 2014 at 9:33 PM, Zhang Haoyu <zhanghy@xxxxxxxxxxx> wrote: >>> 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. -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
