Re: How does kvm support x2apic?

["Zhang Haoyu" <zhanghy@sangfor.com>]

>>>> 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