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

[Julien Grall <julien@xen.org>]

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@amazon.com>
>>>>
>>>> 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@amazon.com>
>>>
>>> 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