linux-kernel - Re: [PATCH v7 01/10] iommu: Add DOMAIN_ATTR_MSI

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Date:	Wed, 20 Apr 2016 17:58:48 +0200
From:	Eric Auger <eric.auger@...aro.org>
To:	Robin Murphy <robin.murphy@....com>, eric.auger@...com,
	alex.williamson@...hat.com, will.deacon@....com, joro@...tes.org,
	tglx@...utronix.de, jason@...edaemon.net, marc.zyngier@....com,
	christoffer.dall@...aro.org, linux-arm-kernel@...ts.infradead.org
Cc:	patches@...aro.org, linux-kernel@...r.kernel.org,
	Bharat.Bhushan@...escale.com, pranav.sawargaonkar@...il.com,
	p.fedin@...sung.com, iommu@...ts.linux-foundation.org,
	Jean-Philippe.Brucker@....com, julien.grall@....com
Subject: Re: [PATCH v7 01/10] iommu: Add DOMAIN_ATTR_MSI_MAPPING attribute

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.

Best Regards

Eric
> 
> Robin.
> 
>> Signed-off-by: Bharat Bhushan <Bharat.Bhushan@...escale.com>
>> Signed-off-by: Eric Auger <eric.auger@...aro.org>
>>
>> ---
>>
>> 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,
>>   };
>>
>>
>