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.
>
> 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