Re: [Xen-devel] AGP problems with 2.0.5

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 13 Mar 2005, at 22:17, Ian Pratt wrote:

> Remap_pfn_range will only work on RAM pages that have been marked
> PageReserved.
> It would be interesting to know what's going on within remap_pte_range.
This is not true -- remap_pfn_range can also be used to map 'invalid 
pages'. That is, those that are entirely outside the normal RAM map.
The problem is that remap_pfn_range is expecting a pseudo-phys address 
range, but most drivers pass a real-phys address range. The fix is 
non-obvious: some callers *do* pass in pseudo-phys address ranges (see, 
for example, packet_mmap in af_packet.c).
I think the correct fix is to extend the Linux interface with something 
like a io_remap_pfn_range, or an extra 'is_ram' flag to 
remap_pfn_range. We then already have (almost) an implementation -- we 
just recycle io_remap_page_range. There are a lot of callers of 
remap_pfn_range, each would need auditing, and we'd have to try to get 
the patch checked in even though it only makes sense when running on 
Xen.
 -- Keir



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