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Re: [PATCH RFCv2 10/15] xen/arm: mm: Allocate xen page tables in domheap rather than xenheap
Hi Stefano,
On 13/05/2021 23:27, Stefano Stabellini wrote:
On Thu, 13 May 2021, Julien Grall wrote:
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.
Got it. Repeating to check if I see the full picture. On ARM64 there are
no changes. On ARM32, at runtime there are no changes mapping/unmapping
pages because xen_map_table is already mapping all pages using domheap,
even xenheap pages are mapped as domheap; so this patch makes no
difference in mapping/unmapping, correct?
For arm32, it makes a slight difference when allocating a new page table
(we didn't call map/unmap before) but this is not called often.
The main "drop" in performance happened when xen_{,map}_table() was
introduced.
The only difference is that on arm32 we are using domheap to allocate
the pages, which is a different (larger) pool.
Yes.
Would you be happy to give you acked-by/reviewed-by on this basis?
Cheers,
--
Julien Grall
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