[Xen-devel] Re: [patch 06/26] Xen-paravirt_ops: paravirt_ops: allocate a fixmap slot

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

Jeremy Fitzhardinge wrote:
> Ingo Molnar wrote:
>   
>> why not vmalloc it on the guest side? fixmaps are bad for this purpose 
>> for a general paravirt implementation, it limits the size of the shared 
>> info page, etc.
>>     
>
> Yes.  vmalloc would have the annoying side-effect of actually allocating
> some pages which would be shadowed by the remapping, but I guess
> get_vm_area would do the job.  I'll give it a go.

Hm, this is a bit awkward.  We need to map the shared info page fairly
early - say, around paging_init - but we're still on the bootmem
allocator at that point, so get_vm_area isn't usable yet.  Using a
fixmap keeps things simple.  It seems to me that having a single fixmap
available is useful for this kind of simple/early mapping, and if
someone needs to map something larger, then they can put it off until
get_vm_area() is available...

    J

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