Re: [Xen-devel] [PATCH v3 4/4] x86/hvm: Indicate avaliability of HW support of APIC virtualization to HVM guests

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 03/13/2014 09:48 PM, Zhang, Yang Z wrote:
> +/*
> + * Leaf 5 (0x40000004)
> + * HVM-specific features
> + */
> +
> +/* EAX Features */
> +#define XEN_HVM_CPUID_APIC_ACCESS_VIRT (1u << 0) /* Virtualized APIC
> registers */
> +#define XEN_HVM_CPUID_X2APIC_VIRT      (1u << 1) /* Virtualized
> x2APIC accesses */
> I still wonder why expose the x2APIC? I guess you only want to know whether the 
> APICv is used and all platforms that support APICv must also support 
> virtualized x2apic. What can guest do if he knows there is only virtuazliaed 
> x2apic but no APICv?
You mean if it has virtualized x2APIC but not virtualized APIC registers 
(i.e. bit 4 in VMCS secondary controls is set but bit 8 is not)? I 
thought that term APICv encompasses both of these features.
Then it's up to the guest to decide whether to use APIC or pirqs:

* If the guest is in APIC mode then it probably should stick to pirqs 
since memory-mapped accesses to APIC will cause VMEXIT.
* If it is in x2APIC mode then it makes sense for it to not use pirqs 
and go with APIC (x2APIC, really) since accesses will be handled 
transparently.
I said in the cover letter I am not particularly happy about having two 
bits for this but I thought it is justified in this case.
-boris

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