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

[Jane Malalane <Jane.Malalane@xxxxxxxxxx>]

On 14/02/2022 13:18, Jan Beulich wrote:
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> 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 ...
> 
So, I was thinking of adding something along the lines of:

+=item B<assisted_xapic=BOOLEAN> B<(x86 only)>
+Enables or disables hardware assisted virtualization for xAPIC. This
+allows accessing APIC registers without a VM-exit. Notice enabling
+this does not guarantee full virtualization for xAPIC, as this can
+only be achieved if hardware supports “APIC-register virtualization”
+and “virtual-interrupt delivery”. The default is settable via
+L<xl.conf(5)>.

and going for assisted_x2apic_available = 
cpu_has_vmx_virtualize_x2apic_mode.

This would prevent the customer from expecting full acceleration when 
apic_register_virt and/or virtual_intr_delivery aren't available whilst 
still offering some if they are not available as Xen currently does. In 
a future patch, we could also expose and add config options for these 
controls if we wanted to.

Thank you for your help,

Jane.