Re: [Xen-devel] [PATCH v7 3/3] x86/hvm: Indicate avaliability of HW support of APIC virtualization to HVM guests

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 03/25/2014 05:45 AM, Jan Beulich wrote:
> > > > On 25.03.14 at 00:18, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > +void vmx_hypervisor_cpuid_leaf(uint32_t sub_idx,
> > +                               uint32_t *eax, uint32_t *ebx,
> > +                               uint32_t *ecx, uint32_t *edx)
> > +{
> > +    if ( sub_idx != 0 )
> > +        return;
> > +
> > +    if ( cpu_has_vmx_apic_reg_virt )
> > +        *eax |= XEN_HVM_CPUID_APIC_ACCESS_VIRT;
> > +    if ( cpu_has_vmx_virtualize_x2apic_mode )
> > +        *eax |= XEN_HVM_CPUID_X2APIC_VIRT;
> > +}
> So did the two of you then settle on (a) needing to expose two bits
> rather than just one and (b) these being the two relevant features
> to expose?
My argument is that we can't know which APIC model a guest uses and so 
both are needed. For PVHVM we default to APIC (MMIO accesses), I can't 
remember what unenlightened HVM Linux would do. And then there are other 
OSs.
For (b) having either (or both) of these two seems to be sufficient to 
bring down the number of VMEXITs when switching from pirqs to APIC. It's 
more important to agree on (a) since for (b) we can always add another bit.
-boris

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