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Re: [Xen-devel] [PATCH v2 0/4] Expose HW APIC virtualization support to HVM guests



On 03/11/2014 03:37 AM, Zhang, Yang Z wrote:
Boris Ostrovsky wrote on 2014-03-11:
Version 2:
* Added ability to specify hypervisor CPUID leaves in config file (this requires
   new sysctl)
* Use 2 bits to indicate what is supported --- one for APIC memory access and
the
   other for x2APIC. Still not sure whether virtual interrupt delivery should be
   exposed as well.

...

I am also not sure whether (cpu_has_vmx_apic_reg_virt &
cpu_has_vmx_virtualize_x2apic_mode) is sufficient to declare full HW
APIC support to a guest. The tests show ~95K VMEXITs when virtual
The former is enough. Hardware has APICv must have the virtualize x2apic too.

interrupt delivery and posted interrupts are turned off so there
appears to still be some benefit. I suppose we can use another CPUID
bit for these two (although I am not particularly eager to do this).
The three features are coexisting. I think you can use one bit to show them.

As I learned yesterday (the hard way) this is not necessarily the case. I was testing this on a laptop that has MSR 0x48b (MSR_IA32_VMX_PROCBASED_CTLS2) return 0x8ff00000000: yes for x2APIC virtualization but no for APIC register access virtualization.

Which is why I added another bit to distinguish the two


Thanks.
-boris


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