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Re: [PATCH RFCv2 10/15] xen/arm: mm: Allocate xen page tables in domheap rather than xenheap

Hi Stefano,

On 12/05/2021 23:44, Stefano Stabellini wrote:
On Sun, 25 Apr 2021, Julien Grall wrote:
From: Julien Grall <jgrall@xxxxxxxxxx>

xen_{un,}map_table() already uses the helper to map/unmap pages
on-demand (note this is currently a NOP on arm64). So switching to
domheap don't have any disavantage.

But this as the benefit:
     - to keep the page tables unmapped if an arch decided to do so
     - reduce xenheap use on arm32 which can be pretty small

Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

Thanks for the patch. It looks OK but let me ask a couple of questions
to clarify my doubts.

This change should have no impact to arm64, right?

For arm32, I wonder why we were using map_domain_page before in
xen_map_table: it wasn't necessary, was it? In fact, one could even say
that it was wrong?
In xen_map_table() we need to be able to map pages from Xen binary, xenheap... On arm64, we would be able to use mfn_to_virt() because everything is mapped in Xen. But that's not the case on arm32. So we need a way to map anything easily.

The only difference between xenheap and domheap are the former is always mapped while the latter may not be. So one can also view a xenheap page as a glorified domheap.

I also don't really want to create yet another interface to map pages (we have vmap(), map_domain_page(), map_domain_global_page()...). So, when I implemented xen_map_table() a couple of years ago, I came to the conclusion that this is a convenient (ab)use of the interface.


Julien Grall



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