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

Hi Jan,

On 5/2/19 8:30 AM, Jan Beulich wrote:
On 02.05.19 at 00:44, <sstabellini@xxxxxxxxxx> wrote:
Hi Jan, I have a question on the PDX code.

The PDX initialization is a bit different between x86 and ARM, but it
follows roughly the same pattern, look at
xen/arch/x86/srat.c:srat_parse_regions (I take that is where things
happen on x86) and xen/arch/arm/setup.c:init_pdx.

Mask is initialized calling pdx_init_mask on a start address, then a
loop fills in the rest of the mask calling pdx_region_mask, based on the
memory regions present.

I wrote a small unit test of the ARM PDX initialization and while I was
playing with addresses and values I noticed that actually if I simply
skip pdx_init_mask and just initialize the mask to 0 (mask = 0) in
init_pdx, the rest of the function always calculates the right mask.

In fact, there are cases where initializing the mask to a value causes
the rest of the code to miss some potential compressions. While
initializing the mask to 0 leads to more optimizations. I can provide
specific examples if you are curious.

Before I make any changes to that code, I would like to understand a bit
better why things are done that way today. Do you know why the mask is
initialized to pdx_init_mask(start-of-ram)?

Well, it is not the start-of-ram on Arm. It is whatever is the start of bank 0. This is because the firmware table (such as DT) may not require ordering and we don't order banks in Xen.

So it may be possible the PDX will not compress if the banks are not ordered in the firmware tables.

I'm confused, and hence I'm perhaps misunderstanding your
question. To me it looks like you're re-asking a question already
answered. On x86 we don't want to squash out any of the low
32 bits, because of the special address ranges that live below
4Gb. Hence we invoke pdx_init_mask(first-block-at-or-above-4Gb).
Note it's not start-of-ram, as you've said.

I think what Stefano is asking is why pdx_init_mask(...) is invoked with the first block address rather than 4GB (or even 0 thought I don't think this is right).

By using the first block address, the PDX will not be able to compress any bits between 0 and the MSB 1' in the first block address. In other word, for a base address 0x200000000 (8GB), the initial mask will be 0x1ffffffff.

Stefano and I were wondering whether it would instead be possible to create the initial mask with pdx_init_mask(4GB) or pdx_init_mask(1GB) (I.e the maximum contiguous range the buddy allocator can allocate).


Julien Grall

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