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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan