Re: [Xen-devel] [PATCH v2] VT-d PI: disable VT-d PI when APICv is disabled

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.06.17 at 11:29, <chao.gao@xxxxxxxxx> wrote:
> @@ -2266,8 +2267,10 @@ int __init intel_vtd_setup(void)
>           * We cannot use posted interrupt if X86_FEATURE_CX16 is
>           * not supported, since we count on this feature to
>           * atomically update 16-byte IRTE in posted format.
> +         * VT-d PI implementation relies on APICv. Thus, disable
> +         * VT-d PI when APICv is disabled.
>           */
> -        if ( !cap_intr_post(iommu->cap) || !cpu_has_cx16 )
> +        if ( !cap_intr_post(iommu->cap) || !cpu_has_cx16 || 
> !opt_apicv_enabled )
>              iommu_intpost = 0;

I'm sorry for thinking of this only now, but why do you check the
command line option here instead of whether APIC-V is actually
in use? I guess there's not going to be hardware supporting VT-d
PI but not APIC-V (albeit one is a chipset feature aiui and the
other a CPU one, so they are kind of independent), but I could
easily imagine hypervisors supporting them entirely independently.
Hence I think you would be better off checking the relevant
secondary exec bit(s).

Jan


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