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

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Date:	Fri, 22 Apr 2016 12:31:04 +0100
From:	Robin Murphy <robin.murphy@....com>
To:	Eric Auger <eric.auger@...aro.org>, 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

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?

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))
		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@...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,
>>>    };
>>>
>>>
>>
>