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

["Chen, Jiqian" <Jiqian.Chen@xxxxxxx>]

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:
>>>> If run Xen with PVH dom0 and hvm domU, hvm will map a pirq for
>>>> a passthrough device by using gsi, see qemu code
>>>> xen_pt_realize->xc_physdev_map_pirq and libxl code
>>>> pci_add_dm_done->xc_physdev_map_pirq. Then xc_physdev_map_pirq
>>>> will call into Xen, but in hvm_physdev_op, PHYSDEVOP_map_pirq
>>>> is not allowed because currd is PVH dom0 and PVH has no
>>>> X86_EMU_USE_PIRQ flag, it will fail at has_pirq check.
>>>>
>>>> So, allow PHYSDEVOP_map_pirq when dom0 is PVH and also allow
>>>> PHYSDEVOP_unmap_pirq for the removal device path to unmap pirq.
>>>> And add a new check to prevent (un)map when the subject domain
>>>> has no X86_EMU_USE_PIRQ flag.
>>>>
>>>> So that the interrupt of a passthrough device can be
>>>> successfully mapped to pirq for domU with X86_EMU_USE_PIRQ flag
>>>> when dom0 is PVH
>>>>
>>>> Signed-off-by: Jiqian Chen <Jiqian.Chen@xxxxxxx>
>>>> Signed-off-by: Huang Rui <ray.huang@xxxxxxx>
>>>> Signed-off-by: Jiqian Chen <Jiqian.Chen@xxxxxxx>
>>>> Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>>>
>>> You keep carrying this R-b, despite making functional changes. This can't be
>>> quite right.
>> Will remove in next version.
>>
>>>
>>> While functionally I'm now okay with the change, I still have a code 
>>> structure
>>> concern:
>>>
>>>> --- 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;

> 
> Jan

-- 
Best regards,
Jiqian Chen.