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Date:   Tue, 22 Jan 2019 10:58:15 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Pierre Morel <pmorel@...ux.ibm.com>
Cc:     Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        Robin Murphy <robin.murphy@....com>,
        gerald.schaefer@...ibm.com, linux-s390@...r.kernel.org,
        walling@...ux.ibm.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] iommu/s390: Declare s390 iommu reserved regions

On Mon, 21 Jan 2019 12:51:14 +0100
Pierre Morel <pmorel@...ux.ibm.com> wrote:

> On 18/01/2019 14:51, Jean-Philippe Brucker wrote:
> > Hi Pierre,
> > 
> > On 18/01/2019 13:29, Pierre Morel wrote:  
> >> On 17/01/2019 14:02, Robin Murphy wrote:  
> >>> On 15/01/2019 17:37, Pierre Morel wrote:  
> >>>> The s390 iommu can only allow DMA transactions between the zPCI device
> >>>> entries start_dma and end_dma.
> >>>>  
> 
> ...
> 
> >>
> >> I already posted a patch retrieving the geometry through
> >> VFIO_IOMMU_GET_INFO using a specific capability for the geometry [1],
> >> and AFAIU, Alex did not agree with this.  
> > 
> > On arm we also need to report the IOMMU geometry to userspace (max IOVA
> > size in particular). Shameer has been working on a solution [2] that
> > presents a unified view of both geometry and reserved regions into the
> > VFIO_IOMMU_GET_INFO call, and I think we should go with that. If I
> > understand correctly it's currently blocked on the RMRR problem and
> > we're waiting for Jacob or Ashok to take a look at it, as Kevin pinged
> > them on thread [1]?
> > 
> > [2] https://lkml.org/lkml/2018/4/18/293
> > 
> > Thanks,
> > Jean
> >   
> 
> Hi Jean,
> 
> I hopped that this proposition went in the same direction based on the 
> following assumptions:
> 
> 
> - The goal of the get_resv_region is defined in iommu.h as:
> -----
> * @get_resv_regions: Request list of reserved regions for a device
> -----
> 
> - A iommu reserve region is a region which should not be mapped.
> Isn't it exactly what happens outside the aperture?
> Shouldn't it be reported by the iommu reserved region?
> 
> - If we use VFIO and want to get all reserved region we will have the 
> VFIO_IOMMU_GET_INFO call provided by Shameer and it can get all reserved 
> regions depending from the iommu driver itself at once by calling the 
> get_reserved_region callback instead of having to merge them with the 
> aperture.
> 
> - If there are other reserved region, depending on the system 
> configuration and not on the IOMMU itself, the VFIO_IOMMU_GET_INFO call 
> will have to merge them with the region gotten from the iommu driver.
> 
> - If we do not use QEMU nor VFIO at all, AFAIU, the standard way to 
> retrieve the reserved regions associated with a device is to call the 
> get_reserved_region callback from the associated iommu.
> 
> Please tell me were I am wrong.

The existing proposal by Shameer exposes an IOVA list, which includes
ranges that the user _can_ map through the IOMMU, therefore this
original patch is unnecessary, the IOMMU geometry is already the
starting point of the IOVA list, creating the original node, which is
split as necessary to account for the reserved regions.  To me,
presenting a user interface that exposes ranges that _cannot_ be mapped
is a strange interface.  If we started from a position of what
information do we want to provide to the user and how will they consume
it, would we present a list of reserved ranges?  I think the only
argument for the negative ranges is that we already have them in the
kernel, but I don't see that it necessarily makes them the right
solution for userspace.


> >> What is different in what you propose?
> >>
> >> @Alex: I was hoping that this patch goes in your direction. What do you
> >> think?

I think it's unnecessary given that in Shameer's proposal:

 - We start from the IOMMU exposed geometry
 - We present a list of usable IOVA ranges, not a list of reserved
   ranges

Thanks,
Alex

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