Re: [Xen-devel] [PATCH] iommu: specify page_count rather than page_order to iommu_map/unmap()...

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.01.19 at 12:56, <Paul.Durrant@xxxxxxxxxx> wrote:
>>  -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: 21 January 2019 11:28
>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
>> Cc: Julien Grall <julien.grall@xxxxxxx>; Andrew Cooper
>> <Andrew.Cooper3@xxxxxxxxxx>; Roger Pau Monne <roger.pau@xxxxxxxxxx>; Wei
>> Liu <wei.liu2@xxxxxxxxxx>; Sander Eikelenboom <linux@xxxxxxxxxxxxxx>;
>> George Dunlap <George.Dunlap@xxxxxxxxxx>; Ian Jackson
>> <Ian.Jackson@xxxxxxxxxx>; Chao Gao <chao.gao@xxxxxxxxx>; Jun Nakajima
>> <jun.nakajima@xxxxxxxxx>; Kevin Tian <kevin.tian@xxxxxxxxx>; Stefano
>> Stabellini <sstabellini@xxxxxxxxxx>; xen-devel <xen-
>> devel@xxxxxxxxxxxxxxxxxxxx>; Konrad Rzeszutek Wilk
>> <konrad.wilk@xxxxxxxxxx>; Tim (Xen.org) <tim@xxxxxxx>
>> Subject: Re: [PATCH] iommu: specify page_count rather than page_order to
>> iommu_map/unmap()...
>> 
>> >>> On 18.01.19 at 17:03, <paul.durrant@xxxxxxxxxx> wrote:
>> > ...and remove alignment assertions.
>> >
>> > Testing shows that certain callers of iommu_legacy_map/unmap() specify
>> > order > 0 ranges that are not order aligned thus causing one of the
>> > IS_ALIGNED() assertions to fire.
>> 
>> As said before - without a much better explanation of why the current
>> order-based model is unsuitable (so far I've been provided only vague
>> pointers into "somewhere in PVH Dom0 boot code" iirc) to understand
>> why it's undesirable to simply make those call sites obey to the current
>> requirements, I'm not happy to see us go this route.
> 
> I thought...
> 
> "Using a count actually makes more sense because the valid
> set of mapping orders is specific to the IOMMU implementation and to it
> should be up to the implementation specific code to translate a mapping
> count into an optimal set of mapping orders (when the code is finally
> modified to support orders > 0)."
> 
> ...was reasonably clear. Is that not a reasonable justification? What else 
> could I say?

Well, I was hoping to be pointed at the (apparently multiple) call sites
where making them match the current function pattern is more involved
and/or less desirable than changing the functions here.

Jan



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