Re: [PATCH v2 1/2] xen+tools: Report Interrupt Controller Virtualization capabilities on x86

[Jane Malalane <Jane.Malalane@xxxxxxxxxx>]

On 10/02/2022 10:03, Roger Pau Monné wrote:
> On Mon, Feb 07, 2022 at 06:21:00PM +0000, Jane Malalane wrote:
>> diff --git a/xen/arch/x86/hvm/vmx/vmcs.c b/xen/arch/x86/hvm/vmx/vmcs.c
>> index 7ab15e07a0..4060aef1bd 100644
>> --- a/xen/arch/x86/hvm/vmx/vmcs.c
>> +++ b/xen/arch/x86/hvm/vmx/vmcs.c
>> @@ -343,6 +343,15 @@ static int vmx_init_vmcs_config(bool bsp)
>>               MSR_IA32_VMX_PROCBASED_CTLS2, &mismatch);
>>       }
>>   
>> +    /* Check whether hardware supports accelerated xapic and x2apic. */
>> +    if ( bsp )
>> +    {
>> +        assisted_xapic_available = cpu_has_vmx_virtualize_apic_accesses;
>> +        assisted_x2apic_available = (cpu_has_vmx_apic_reg_virt ||
>> +                                     cpu_has_vmx_virtual_intr_delivery) &&
>> +                                    cpu_has_vmx_virtualize_x2apic_mode;
> 
> I've been think about this, and it seems kind of asymmetric that for
> xAPIC mode we report hw assisted support only with
> virtualize_apic_accesses available, while for x2APIC we require
> virtualize_x2apic_mode plus either apic_reg_virt or
> virtual_intr_delivery.
> 
> I think we likely need to be more consistent here, and report hw
> assisted x2APIC support as long as virtualize_x2apic_mode is
> available.
> 
> This will likely have some effect on patch 2 also, as you will have to
> adjust vmx_vlapic_msr_changed.
> 
> Thanks, Roger.

Any other thoughts on this? As on one hand it is asymmetric but also 
there isn't much assistance with only virtualize_x2apic_mode set as, in 
this case, a VM exit will be avoided only when trying to access the TPR 
register.

Thanks,

Jane.