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

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

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);

> 
> Jan

-- 
Best regards,
Jiqian Chen.