Re: [XEN PATCH v11 2/8] x86/pvh: Allow (un)map_pirq when dom0 is PVH

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.07.2024 04:56, Chen, Jiqian wrote:
> On 2024/7/2 16:44, Jan Beulich wrote:
>> On 02.07.2024 05:15, Chen, Jiqian wrote:
>>> On 2024/7/1 15:44, Jan Beulich wrote:
>>>> On 30.06.2024 14:33, Jiqian Chen wrote:
>>>>> --- a/xen/arch/x86/physdev.c
>>>>> +++ b/xen/arch/x86/physdev.c
>>>>> @@ -323,6 +323,13 @@ ret_t do_physdev_op(int cmd, 
>>>>> XEN_GUEST_HANDLE_PARAM(void) arg)
>>>>>          if ( !d )
>>>>>              break;
>>>>>  
>>>>> +        /* Prevent mapping when the subject domain has no 
>>>>> X86_EMU_USE_PIRQ */
>>>>> +        if ( is_hvm_domain(d) && !has_pirq(d) )
>>>>> +        {
>>>>> +            rcu_unlock_domain(d);
>>>>> +            return -EOPNOTSUPP;
>>>>> +        }
>>>>> +
>>>>>          ret = physdev_map_pirq(d, map.type, &map.index, &map.pirq, &msi);
>>>>>  
>>>>>          rcu_unlock_domain(d);
>>>>> @@ -346,6 +353,13 @@ ret_t do_physdev_op(int cmd, 
>>>>> XEN_GUEST_HANDLE_PARAM(void) arg)
>>>>>          if ( !d )
>>>>>              break;
>>>>>  
>>>>> +        /* Prevent unmapping when the subject domain has no 
>>>>> X86_EMU_USE_PIRQ */
>>>>> +        if ( is_hvm_domain(d) && !has_pirq(d) )
>>>>> +        {
>>>>> +            rcu_unlock_domain(d);
>>>>> +            return -EOPNOTSUPP;
>>>>> +        }
>>>>> +
>>>>>          ret = physdev_unmap_pirq(d, unmap.pirq);
>>>>>  
>>>>>          rcu_unlock_domain(d);
>>>>
>>>> If you did go look, you will have noticed that we use "return" in the 
>>>> middle
>>>> of this function only very sparingly (when alternatives would result in 
>>>> more
>>>> complicated code elsewhere). I think you want to avoid "return" here, too,
>>>> and probably go even further and avoid the extra rcu_unlock_domain() as 
>>>> well.
>>>> That's easily possible to arrange for (taking the latter case as example):
>>>>
>>>>         /* Prevent unmapping when the subject domain has no 
>>>> X86_EMU_USE_PIRQ */
>>>>         if ( !is_hvm_domain(d) || has_pirq(d) )
>>>>             ret = physdev_unmap_pirq(d, unmap.pirq);
>>>>         else
>>>>             ret = -EOPNOTSUPP;
>>>>
>>>>         rcu_unlock_domain(d);
>>>>
>>>> Personally I would even use a conditional operator here, but I believe
>>>> others might dislike its use in situations like this one.
>>>>
>>>> The re-arrangement make a little more noticeable though that the comment
>>>> isn't quite right either: PV domains necessarily have no
>>>> X86_EMU_USE_PIRQ. Maybe "... has no notion of pIRQ"?
>>>
>>> Or just like below?
>>>
>>>         /*
>>>          * Prevent unmapping when the subject hvm domain has no
>>>          * X86_EMU_USE_PIRQ
>>>          */
>>>         if ( is_hvm_domain(d) && !has_pirq(d) )
>>>             ret = -EOPNOTSUPP;
>>>         else
>>>             ret = physdev_unmap_pirq(d, unmap.pirq);
>>
>> No objection to the slightly changed comment. The code alternative you
>> present is of course functionally identical, yet personally I prefer to
>> have the "good" case on the "if" branch and the "bad" one following
>> "else". I wouldn't insist, though.
> OK, will change "good" case on the "if" branch.
> Do I need to change "!is_hvm_domain(d)" to "is_pv_domain(d)" ?
> And then have:
> 
>         /* Only unmapping when the subject domain has a notion of PIRQ */
>         if ( is_pv_domain(d) || has_pirq(d) )
>             ret = physdev_unmap_pirq(d, unmap.pirq);
>         else
>             ret = -EOPNOTSUPP;

I for one would prefer if you kept using is_hvm_domain(), for being more
precise in this situation.

Jan