Re: [Qemu-devel] [RFC PATCH] vfio: VFIO Driver core framework

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 2011-11-29 at 17:20 -0600, Stuart Yoder wrote:
> >
> > BTW, github now has updated trees:
> >
> > git://github.com/awilliam/linux-vfio.git vfio-next-20111129
> > git://github.com/awilliam/qemu-vfio.git vfio-ng
> 
> Hi Alex,
> 
> Have been looking at vfio a bit.   A few observations and things
> we'll need to figure out as it relates to the Freescale iommu.
> 
> __vfio_dma_map() assumes that mappings are broken into
> 4KB pages.   That will not be true for us.   We normally will be mapping
> much larger physically contiguous chunks for our guests.  Guests will
> get hugetlbfs backed memory with very large pages (e.g. 16MB,
> 64MB) or very large chunks allocated by some proprietary
> means.

Hi Stuart,

I think practically everyone has commented on the 4k mappings ;)  There
are a few problems around this.  The first is that iommu drivers don't
necessarily support sub-region unmapping, so if we map 1GB and later
want to unmap 4k, we can't do it atomically.  4k gives us the most
flexibility for supporting fine granularities.  Another problem is that
we're using get_user_pages to pin memory.  It's been suggested that we
should use mlock for this, but I can't find anything that prevents a
user from later munlock'ing the memory and then getting access to memory
they shouldn't have.  Those kind of limit us, but I don't see it being
an API problem for VFIO, just implementation.

> Also, mappings will have additional Freescale-specific attributes
> that need to get passed through to dma_map somehow.   For
> example, the iommu can stash directly into a CPU's cache
> and we have iommu mapping properties like the cache stash
> target id and an operation mapping attribute.
> 
> How do you envision handling proprietary attributes
> in struct vfio_dma_map?

Let me turn the question around, how do you plan to support proprietary
attributes in the IOMMU API?  Is the user level the appropriate place to
specify them, or are they an intrinsic feature of the domain?  We've
designed struct vfio_dma_map for extension, so depending on how many
bits you need, we can make a conduit using the flags directly or setting
a new flag to indicate presence of an arch specific attributes field.
Thanks,

Alex