Re: [Xen-devel] [PATCH] xen/arm: skip first page when RAM starts at 0x0

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.04.19 at 23:27, <Julien.Grall@xxxxxxx> wrote:
> On 25/04/2019 18:51, Stefano Stabellini wrote:
>> Xen assumes that RAM starts at addresses greater than 0x0 in a few
>> places. The PDX code is one of them but there are more.
> 
> A bit more context in the commit message would have been useful. Imagine 
> if we have to look at the commit message it in 2 years time.
> 
>> 
>> For now, skip the first page in memory when the start addess is 0x0.
>> 
>> Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>

Isn't this in response to a bug report / completely different
code change from someone at DornerWorks? In which case
shouldn't there be a Reported-by?

> For a first we need to gather feedback from contributors that know a bit 
> more of the code that may be affected. AFAICT, there are only two pieces 
> where we hand over memory to common code:
>      - PDX: The problem is passing 0 to pdx_init_mask() will result to a 
> ~0 mask defeating the compression later on.

On x86 the function gets called only for memory blocks above 4Gb.
Question is whether on Arm you also have some ad hoc boundary
below which there's no point to look for bits to compact. If not I
wonder why you call the function at all; at the very least (as you
seem to imply) it shouldn't be called when
bootinfo.mem.bank[0].start is zero.

>      - Buddy allocator: Jan has been suggesting to stick a check in 
> init_xenheap_pages(). This would go the other ugliness existing to deal 
> with the buddy allocator.

And this would then also take care of future architectures Xen may
get ported to. (No idea what other ugliness you refer to.)

Jan



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