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

[David Vrabel <david.vrabel@citrix.com>]

On 11/03/14 03:53, Boris Ostrovsky wrote:
> 
> 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.
> 
> 
> 
> HVM guests running on HW that supports HW APIC virtualization features
> (APIC-register virtualization, virtual interrupt delivery, etc) may
> want to use APIC instead of hvm_pirqs. Since we are not guaranteed to
> have these features on VMX (for example, there is a boot option to
> turn it off) and there is no such support on SVM we need to make the
> guest aware that its APIC accesses may not be so bad.
> 
> CPUID seems to be a good way to provide this info to the guest.

It's seems a bit odd to use a Xen-specific (?) CPUID leaf to report that
the guest should use a bare-metal feature -- that's the default
behaviour of a HVM guest.

I think it makes more sense to hint that the guest that a PV alternate
is available and then omit this hint if using a virtualized APIC
performs better.

David