Re: [PATCH v5 2/2] x86/xen: Allow per-domain usage of hardware virtualized APIC

[Jane Malalane <Jane.Malalane@xxxxxxxxxx>]

On 08/03/2022 14:41, Jan Beulich wrote:
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> On 08.03.2022 15:31, Jane Malalane wrote:
>> On 08/03/2022 12:33, Roger Pau Monné wrote:
>>> On Tue, Mar 08, 2022 at 01:24:23PM +0100, Jan Beulich wrote:
>>>> On 08.03.2022 12:38, Roger Pau Monné wrote:
>>>>> On Mon, Mar 07, 2022 at 03:06:09PM +0000, Jane Malalane wrote:
>>>>>> @@ -685,13 +687,31 @@ int arch_sanitise_domain_config(struct 
>>>>>> xen_domctl_createdomain *config)
>>>>>>            }
>>>>>>        }
>>>>>>    
>>>>>> -    if ( config->arch.misc_flags & ~XEN_X86_MSR_RELAXED )
>>>>>> +    if ( config->arch.misc_flags & ~(XEN_X86_MSR_RELAXED |
>>>>>> +                                     XEN_X86_ASSISTED_XAPIC |
>>>>>> +                                     XEN_X86_ASSISTED_X2APIC) )
>>>>>>        {
>>>>>>            dprintk(XENLOG_INFO, "Invalid arch misc flags %#x\n",
>>>>>>                    config->arch.misc_flags);
>>>>>>            return -EINVAL;
>>>>>>        }
>>>>>>    
>>>>>> +    if ( (assisted_xapic || assisted_x2apic) && !hvm )
>>>>>> +    {
>>>>>> +        dprintk(XENLOG_INFO,
>>>>>> +                "Interrupt Controller Virtualization not supported for 
>>>>>> PV\n");
>>>>>> +        return -EINVAL;
>>>>>> +    }
>>>>>> +
>>>>>> +    if ( (assisted_xapic && !assisted_xapic_available) ||
>>>>>> +         (assisted_x2apic && !assisted_x2apic_available) )
>>>>>> +    {
>>>>>> +        dprintk(XENLOG_INFO,
>>>>>> +                "Hardware assisted x%sAPIC requested but not 
>>>>>> available\n",
>>>>>> +                assisted_xapic && !assisted_xapic_available ? "" : "2");
>>>>>> +        return -EINVAL;
>>>>>
>>>>> I think for those two you could return -ENODEV if others agree.
>>>>
>>>> If by "two" you mean the xAPIC and x2APIC aspects here (and not e.g. this
>>>> and the earlier if()), then I agree. I'm always in favor of using distinct
>>>> error codes when possible and at least halfway sensible.
>>>
>>> I would be fine by using it for the !hvm if also. IMO it makes sense
>>> as PV doesn't have an APIC 'device' at all, so ENODEV would seem
>>> fitting. EINVAL is also fine as the caller shouldn't even attempt that
>>> in the first place.
>>>
>>> So let's use it for the last if only.
>> Wouldn't it make more sense to use -ENODEV particularly for the first? I
>> agree that -ENODEV should be reported in the first case because it
>> doesn't make sense to request acceleration of something that doesn't
>> exist and I should have put that. But having a look at the hap code
>> (since it resembles the second case), it returns -EINVAL when it is not
>> available, unless you deem this to be different or, in retrospective,
>> that the hap code should too have been coded to return -ENODEV.
>>
>> if ( hap && !hvm_hap_supported() )
>>       {
>>           dprintk(XENLOG_INFO, "HAP requested but not available\n");
>>           return -EINVAL;
>>       }
> 
> This is just one of the examples where using -ENODEV as you suggest
> would introduce an inconsistency. We use -EINVAL also for other
> purely guest-type dependent checks.
> 
> Jan
Hi Jan, so here I was comparing the hap implementation with the second 
case, i.e.

if ( (assisted_xapic && !assisted_xapic_available) ||
      (assisted_x2apic && !assisted_x2apic_available) )

and you seem to agree that using -ENODEV would be inconsistent? Have I 
misinterpreted this?

Thanks,

Jane.