Re: [PATCH v7 01/10] iommu: Add DOMAIN_ATTR_MSI_MAPPING attribute

[Eric Auger]

Hi Robin,
On 04/22/2016 01:31 PM, Robin Murphy wrote:
> On 20/04/16 16:58, Eric Auger wrote:
>> Hi Robin,
>> On 04/20/2016 02:47 PM, Robin Murphy wrote:
>>> Hi Eric,
>>>
>>> On 19/04/16 17:56, Eric Auger wrote:
>>>> Introduce a new DOMAIN_ATTR_MSI_MAPPING domain attribute. If supported,
>>>> this means the MSI addresses need to be mapped in the IOMMU.
>>>>
>>>> x86 IOMMUs typically don't expose the attribute since on x86, MSI write
>>>> transaction addresses always are within the 1MB PA region [FEE0_0000h -
>>>> FEF0_000h] window which directly targets the APIC configuration
>>>> space and
>>>> hence bypass the sMMU. On ARM and PowerPC however MSI transactions are
>>>> conveyed through the IOMMU.
>>>
>>> What's stopping us from simply inferring this from the domain's IOMMU
>>> not advertising interrupt remapping capabilities?
>> My current understanding is it is not possible:
>> on x86 CAP_INTR_REMAP is not systematically exposed (the feature can be
>> disabled) and MSIs are never mapped in the IOMMU I think.
> 
> Not sure I follow - if the feature is disabled such that the IOMMU
> doesn't isolate MSIs, then it's no different a situation from the SMMU, no?

sorry I understood you wanted to use IOMMU_CAP_INTR_REMAP as the sole
criteria to detect whether MSI mapping was requested.
> 
> My point was that this logic:
> 
>     if (IOMMU_CAP_INTR_REMAP)
>         we're good
>     else if (DOMAIN_ATTR_MSI_MAPPING)
>         if (acquire_msi_remapping_resources(domain))
>             we're good
>         else
>             oh no!
>     else
>         oh no!
> 
> should be easily reducible to this:
> 
>     if (IOMMU_CAP_INTR_REMAP)
>         we're good
>     else if (acquire_msi_remapping_resources(domain))

But Can't we imagine a mix of smmus on the same platform, some
requesting MSI mapping and some which don't. As soon as an smmu requires
MSI mapping, CONFIG_IOMMU_DMA_RESERVED is set and
acquire_msi_remapping_resources(domain) will be implemented & succeed.
Doesn't it lead to a wrong decision. Do I miss something, or do you
consider this situation as far-fetched?

Thanks

Eric

>         we're good
>     else
>         oh no!    // Don't care whether the domain ran out of
>             // resources or simply doesn't support it,
>             // either way we can't proceed.
> 
> Robin.
> 
>> Best Regards
>>
>> Eric
>>>
>>> Robin.
>>>
>>>> Signed-off-by: Bharat Bhushan <Bharat.Bhushan@xxxxxxxxxxxxx>
>>>> Signed-off-by: Eric Auger <eric.auger@xxxxxxxxxx>
>>>>
>>>> ---
>>>>
>>>> v4 -> v5:
>>>> - introduce the user in the next patch
>>>>
>>>> RFC v1 -> v1:
>>>> - the data field is not used
>>>> - for this attribute domain_get_attr simply returns 0 if the
>>>> MSI_MAPPING
>>>>     capability if needed or <0 if not.
>>>> - removed struct iommu_domain_msi_maps
>>>> ---
>>>>    include/linux/iommu.h | 1 +
>>>>    1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
>>>> index 62a5eae..b3e8c5b 100644
>>>> --- a/include/linux/iommu.h
>>>> +++ b/include/linux/iommu.h
>>>> @@ -113,6 +113,7 @@ enum iommu_attr {
>>>>        DOMAIN_ATTR_FSL_PAMU_ENABLE,
>>>>        DOMAIN_ATTR_FSL_PAMUV1,
>>>>        DOMAIN_ATTR_NESTING,    /* two stages of translation */
>>>> +    DOMAIN_ATTR_MSI_MAPPING, /* Require MSIs mapping in iommu */
>>>>        DOMAIN_ATTR_MAX,
>>>>    };
>>>>
>>>>
>>>
>>
>