Re: [RFC v3 07/15] iommu: iommu_get/put_single_reserved

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

Hi Marc,
On 02/18/2016 05:51 PM, Marc Zyngier wrote:
> On 18/02/16 16:42, Eric Auger wrote:
>> Hello,
>> On 02/18/2016 12:06 PM, Marc Zyngier wrote:
>>> On Fri, 12 Feb 2016 08:13:09 +0000
>>> Eric Auger <eric.auger-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> wrote:
>>>
>>>> This patch introduces iommu_get/put_single_reserved.
>>>>
>>>> iommu_get_single_reserved allows to allocate a new reserved iova page
>>>> and map it onto the physical page that contains a given physical address.
>>>> It returns the iova that is mapped onto the provided physical address.
>>>> Hence the physical address passed in argument does not need to be aligned.
>>>>
>>>> In case a mapping already exists between both pages, the IOVA mapped
>>>> to the PA is directly returned.
>>>>
>>>> Each time an iova is successfully returned a binding ref count is
>>>> incremented.
>>>>
>>>> iommu_put_single_reserved decrements the ref count and when this latter
>>>> is null, the mapping is destroyed and the iova is released.
>>>
>>> I wonder if there is a requirement for the caller to find out about the
>>> size of the mapping, or to impose a given size... MSIs clearly do not
>>> have that requirement (this is always a 32bit value), but since. 
>>> allocations usually pair address and size, I though I'd ask...
>> Yes. Currently this only makes sure the host PA is mapped and returns
>> the corresponding IOVA. It is part of the discussion we need to have on
>> the API besides the problematic of which API it should belong to.
> 
> One of the issues I have with the API at the moment is that there is no
> control on the page size. Imagine you have allocated a 4kB IOVA window
> for your MSI, but your IOMMU can only map 64kB (not unreasonable to
> imagine on arm64). What happens then?
The code checks the IOVA window size is aligned with the IOMMU page size
so I think that case is handled at iova domain creation
(arm_smmu_alloc_reserved_iova_domain).
> 
> Somehow, userspace should be told about it, one way or another.
I agree on that point. The user-space should be provided with the
information about the requested iova pool size and alignments. This is
missing in current rfc series.

Best Regards

Eric
> 
> Thanks,
> 
> 	M.
>