linux-kernel - Re: [PATCH 1/2] irqdomain: add empty irq_domain_check_msi

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Date:   Thu, 2 Mar 2017 14:31:37 +0100
From:   Mian Yousaf Kaukab <yousaf.kaukab@...e.com>
To:     Auger Eric <eric.auger@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, marc.zyngier@....com,
        alex.williamson@...hat.com
Cc:     will.deacon@....com
Subject: Re: [PATCH 1/2] irqdomain: add empty irq_domain_check_msi_remap

On 03/02/2017 02:12 PM, Auger Eric wrote:
> Hi Yousaf,
> 
> On 02/03/2017 13:23, Mian Yousaf Kaukab wrote:
>> On 03/02/2017 11:24 AM, Auger Eric wrote:
>>> Hi Mian Yousaf,
>>>
>>> On 02/03/2017 11:01, Mian Yousaf Kaukab wrote:
>>>> Fix following build error for s390:
>>>> drivers/vfio/vfio_iommu_type1.c: In function
>>>> 'vfio_iommu_type1_attach_group':
>>>> drivers/vfio/vfio_iommu_type1.c:1290:25: error: implicit declaration
>>>> of function 'irq_domain_check_msi_remap'
>>>>
>>>> Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@...e.com>
>>>> ---
>>>>    include/linux/irqdomain.h | 4 ++++
>>>>    1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/include/linux/irqdomain.h b/include/linux/irqdomain.h
>>>> index 188eced6813e..137817b08cdc 100644
>>>> --- a/include/linux/irqdomain.h
>>>> +++ b/include/linux/irqdomain.h
>>>> @@ -524,6 +524,10 @@ static inline struct irq_domain
>>>> *irq_find_matching_fwnode(
>>>>    {
>>>>        return NULL;
>>>>    }
>>>> +static inline bool irq_domain_check_msi_remap(void)
>>>> +{
>>>> +    return true;
>>> By default you should rather return false, reporting there is no MSI
>>> remapping capability on irq domain side. Besides thank you for the fix.
>> I choose to return true based on the function header comments of
>> irq_domain_check_msi_remap. It says
>>
>> "Return: false if any MSI irq domain does not support IRQ remapping,
>> true otherwise (including if there is no MSI irq domain)"
>>
>> So function should return true in case of no MSI irq domain. Have I miss
>> understood this?
> This behavior is indeed mandated on ARM - where MSI are translated by
> the smmu - to allow safe device assignment if there is no MSI domain,
> ie. in this situation there is no risk an assigned device writes into an
> MSI doorbell.
> 
> As the function is not implemented at all in your case, personally I
> would rather be defensive though and return false. You were not able to
> check the capability.
OK Agree. I will send an update as soon as a decision is made on 2/2.

BR,
Yousaf