Re: [Xen-devel] __vmap multiple times same mfn

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

On 02/28/2017 10:51 PM, Andrew Cooper wrote:
> On 28/02/17 19:50, Oleksandr Andrushchenko wrote:
> > Hi, all!
> >
> > I have a use-case when I may need to call __vmap for kernel provided
> > IPAs (read MFNs)
> >
> > which may not be PAGE_SIZE aligned etc.
> >
> > The question is if it is safe to call __vmap multiple times for
> > different IPAs
> >
> > sharing the same page (mfn), e.g. map something like 6ca00 0080 and
> > 6ca00 00a0?
> What are you trying to do?
Well, the use-case is as follows: say, there are 2 structures I want to 
access
(S1 and S2), S1 occupying pages A'-B-C' and S2 in pages C'-D'
(I mark with apostrophe here partially occupied pages, e.g. page A is
partially occupied and B is fully used by S1)
No guarantee how pages A:D are located in memory
So, for that reason I want to __vmap A-B-C to access S1 and C-D to access S2
> You specifically can use vmap to make non-contiguous MFNs end up
> contiguous in virtual address space.
this is what I want - see above
> The mappings themselves are of course on page boundaries,
Of course I have no intention to *map* on non page boundary,
but I want to *access* non page aligned data
> so you can't
> make a non-page-aligned thing appear aligned.
I am not trying to align, I use void* from __vmap
and add offset to S1/S2 start in the example above, e.g.
S1 = __vmap(A,B,C) + offset_in_page(S1)
> ~Andrew
So, the question remains: if I can __vmap page C for S1 and also
__vmap it for S2

Thank you,
Oleksandr

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