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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.02.2022 14:11, Jane Malalane wrote:
> On 11/02/2022 11:46, Jan Beulich wrote:
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>> On 11.02.2022 12:29, Roger Pau Monné wrote:
>>> On Fri, Feb 11, 2022 at 10:06:48AM +0000, Jane Malalane wrote:
>>>> 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.
>>>
>>> I've been thinking about this, and reporting hardware assisted
>>> x{2}APIC virtualization with just
>>> SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES or
>>> SECONDARY_EXEC_VIRTUALIZE_X2APIC_MODE doesn't seem very helpful. While
>>> those provide some assistance to the VMM in order to handle APIC
>>> accesses, it will still require a trap into the hypervisor to handle
>>> most of the accesses.
>>>
>>> So maybe we should only report hardware assisted support when the
>>> mentioned features are present together with
>>> SECONDARY_EXEC_APIC_REGISTER_VIRT?
>>
>> Not sure - "some assistance" seems still a little better than none at all.
>> Which route to go depends on what exactly we intend the bit to be used for.
>>
> True. I intended this bit to be specifically for enabling 
> assisted_x{2}apic. So, would it be inconsistent to report hardware 
> assistance with just VIRTUALIZE_APIC_ACCESSES or VIRTUALIZE_X2APIC_MODE 
> but still claim that x{2}apic is virtualized if no MSR accesses are 
> intercepted with XEN_HVM_CPUID_X2APIC_VIRT (in traps.c) so that, as you 
> say, the guest gets at least "some assistance" instead of none but we 
> still claim x{2}apic virtualization when it is actually complete? Maybe 
> I could also add a comment alluding to this in the xl documentation.

To rephrase my earlier point: Which kind of decisions are the consumer(s)
of us reporting hardware assistance going to take? In how far is there a
risk that "some assistance" is overall going to lead to a loss of
performance? I guess I'd need to see comment and actual code all in one
place ...

Jan