Re: [Qemu-devel] [PATCH 2/3] exec: simplify address_space_get_iotlb_entry

[David Gibson]

On Tue, Jun 06, 2017 at 04:34:30PM +0200, Paolo Bonzini wrote:
> 
> 
> On 05/06/2017 05:07, Peter Xu wrote:
> > I don't sure whether it'll be a good interface for IOTLB. AFAIU at
> > least for VT-d, the IOMMU translation is page aligned which is defined
> > by spec, so it makes sense that (again at least for VT-d) here we'd
> > better just use page_mask/addr_mask.
> > 
> > That's also how I know about IOMMU in general - I assume it do the
> > translations always with page masks (never arbitary length), though
> > page size can differ from platfrom to platform, that's why here the
> > IOTLB interface used addr_mask, then it works for all platforms. I
> > don't know whether I'm 100% correct here though.
> > 
> > Maybe David/Paolo/... would comment as well?
> 
> I would ask David.  There are PowerPC MMUs that allow fast lookup of
> arbitrarily-sized windows (not necessarily power of two),

Uh.. I'm not sure what you mean here.  You might be thinking of the
BATs which really old (32-bit) PowerPC MMUs had - those allow
arbitrary large block translations, but they do have to be a power of
two.

> so maybe the
> IOMMUs can do the same.

The only Power IOMMU I know about uses a fixed, power-of-two page size
per DMA window.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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