Re: [PATCH v7 6/7] iommu/arm-smmu: do not advertise IOMMU_CAP_INTR_REMAP

[Eric Auger <eric.auger-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

Hi Robin,
On 04/22/2016 01:16 PM, Robin Murphy wrote:
> Hi Eric, Alex,
> 
> On 19/04/16 18:24, Eric Auger wrote:
>> Do not advertise IOMMU_CAP_INTR_REMAP for arm-smmu(-v3). Indeed the
>> irq_remapping capability is abstracted on irqchip side for ARM as
>> opposed to Intel IOMMU featuring IRQ remapping HW.
>>
>> So to check IRQ remapping capability, the msi domain needs to be
>> checked instead.
>>
>> This commit needs to be applied after "vfio/type1: also check IRQ
>> remapping capability at msi domain" else the legacy interrupt
>> assignment gets broken with arm-smmu.
> 
> Hmm, that smells of papering over a different problem. I may have missed
> it, but I don't see anything changing legacy interrupt behaviour in this
> series - are legacy INTx (or platform) interrupts intrinsically safe
> because they're physically wired, or intrinsically unsafe because they
> could be shared?

I think it is safe. With legacy/platform interrupts we have:
vfio pci driver physical IRQ handler signals an irqfd.
upon this irqfd signaling KVM injects a virtual IRQ.

So the assigned device does not have any way to trigger a storm of
interrupts on the host, as opposed to with MSI.

Does it make sense to you?

Best Regards

Eric

 If it's the latter then I don't see how the IOMMU or
> MSI controller changes anything in that respect, and if it's the former
> then surely we should support that right now without the SMMU having to
> lie about MSI isolation? I started looking into it but I'm a bit lost...
> 
> Robin.
> 
>> Signed-off-by: Eric Auger <eric.auger-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
>> ---
>>   drivers/iommu/arm-smmu-v3.c | 3 ++-
>>   drivers/iommu/arm-smmu.c    | 3 ++-
>>   2 files changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
>> index afd0dac..1d0106c 100644
>> --- a/drivers/iommu/arm-smmu-v3.c
>> +++ b/drivers/iommu/arm-smmu-v3.c
>> @@ -1386,7 +1386,8 @@ static bool arm_smmu_capable(enum iommu_cap cap)
>>       case IOMMU_CAP_CACHE_COHERENCY:
>>           return true;
>>       case IOMMU_CAP_INTR_REMAP:
>> -        return true; /* MSIs are just memory writes */
>> +        /* interrupt translation handled at MSI controller level */
>> +        return false;
>>       case IOMMU_CAP_NOEXEC:
>>           return true;
>>       default:
>> diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
>> index 492339f..6232b2a 100644
>> --- a/drivers/iommu/arm-smmu.c
>> +++ b/drivers/iommu/arm-smmu.c
>> @@ -1312,7 +1312,8 @@ static bool arm_smmu_capable(enum iommu_cap cap)
>>            */
>>           return true;
>>       case IOMMU_CAP_INTR_REMAP:
>> -        return true; /* MSIs are just memory writes */
>> +        /* interrupt translation handled at MSI controller level */
>> +        return false;
>>       case IOMMU_CAP_NOEXEC:
>>           return true;
>>       default:
>>
>