linux-kernel - Re: [RFC v2 8/8] iommu/arm-smmu: implement add_reserved

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Date:   Fri, 11 Nov 2016 17:45:19 +0100
From:   Auger Eric <eric.auger@...hat.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     drjones@...hat.com, jason@...edaemon.net, kvm@...r.kernel.org,
        marc.zyngier@....com, punit.agrawal@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        diana.craciun@....com, alex.williamson@...hat.com,
        pranav.sawargaonkar@...il.com,
        linux-arm-kernel@...ts.infradead.org, tglx@...utronix.de,
        robin.murphy@....com, christoffer.dall@...aro.org,
        eric.auger.pro@...il.com
Subject: Re: [RFC v2 8/8] iommu/arm-smmu: implement add_reserved_regions
 callback

Hi Joerg,

On 11/11/2016 17:22, Joerg Roedel wrote:
> Hi Eric,
> 
> On Fri, Nov 11, 2016 at 04:47:01PM +0100, Auger Eric wrote:
>> Effectively in passthrough use case, the userspace defines the address
>> space layout and maps guest RAM PA=IOVA to PAs (using
>> VFIO_IOMMU_MAP_DMA). But this address space does not comprise the MSI
>> IOVAs. Userspace does not care about MSI IOMMU mapping. So the MSI IOVA
>> region must be allocated by either the VFIO driver or the IOMMU driver I
>> think. Who else could initialize the IOVA allocator domain?
> 
> So I think we need a way to tell userspace about the reserved regions
> (per iommu-group) so that userspace knows where it can not map anything,
Current plan is to expose that info through an iommu-group sysfs
attribute, as you and Robin advised.
> and VFIO can enforce that. But the right struct here is not an
> iova-allocator rb-tree, a ordered linked list should be sufficient.
I plan a linked list to store the reserved regions (P2P regions, MSI
region, ...). get_dma_regions is called with a list local to a function
for that. Might be needed to move that list head in the iommu_group to
avoid calling the get_dm_regions again in the attribute show function?

But to allocate the IOVAs within the MSI reserved region, I understand
you don't want us to use the iova.c allocator, is that correct? We need
an allocator though, even a very basic one based on bitmap or whatever.
There potentially have several different physical MSI frame pages to map.

Best Regards

Eric

> 
> 
> 	Joerg
>