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

[Chao Gao <chao.gao@xxxxxxxxx>]

On Wed, Jun 07, 2017 at 06:31:18AM -0600, Jan Beulich wrote:
>>>> 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).

Those relevant structure hasn't been initialized here. But your
thought is definitely right. How about clear iommu_intpost in
vmx_init_vmcs_config() when finding cpu-side interrupt posting
can't be enabled?

Thanks
Chao

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