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From: Julien Grall <julien@xen.org>
To: Stefano Stabellini <sstabellini@kernel.org>
Cc: xen-devel@lists.xenproject.org, Wei.Chen@arm.com,
	Henry.Wang@arm.com, Penny.Zheng@arm.com,
	Bertrand.Marquis@arm.com, Julien Grall <jgrall@amazon.com>,
	Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
Subject: Re: [PATCH RFCv2 10/15] xen/arm: mm: Allocate xen page tables in domheap rather than xenheap
Date: Sat, 15 May 2021 09:48:32 +0100	[thread overview]
Message-ID: <602bea61-9db2-a27d-1fed-001e2b5b2176@xen.org> (raw)
In-Reply-To: <alpine.DEB.2.21.2105131522030.5018@sstabellini-ThinkPad-T480s>

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


  reply	other threads:[~2021-05-15  8:48 UTC|newest]

Thread overview: 58+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-25 20:13 [PATCH RFCv2 00/15] xen/arm: mm: Remove open-coding mappings Julien Grall
2021-04-25 20:13 ` [PATCH RFCv2 01/15] xen/arm: lpae: Rename LPAE_ENTRIES_MASK_GS to LPAE_ENTRY_MASK_GS Julien Grall
2021-05-11 22:12   ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 02/15] xen/arm: lpae: Use the generic helpers to defined the Xen PT helpers Julien Grall
2021-05-11 22:26   ` Stefano Stabellini
2021-05-12 14:26     ` Julien Grall
2021-05-12 21:30       ` Stefano Stabellini
2021-05-12 22:16         ` Julien Grall
2021-05-13 22:44           ` Stefano Stabellini
2021-07-03 17:32             ` Julien Grall
2021-07-13 20:53               ` Stefano Stabellini
2021-07-14 17:40                 ` Julien Grall
2021-04-25 20:13 ` [PATCH RFCv2 03/15] xen/arm: p2m: Replace level_{orders, masks} arrays with LEVEL_{ORDER, MASK} Julien Grall
2021-05-11 22:33   ` Stefano Stabellini
2021-05-12 14:39     ` Julien Grall
2021-04-25 20:13 ` [PATCH RFCv2 04/15] xen/arm: mm: Allow other mapping size in xen_pt_update_entry() Julien Grall
2021-05-11 22:42   ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 05/15] xen/arm: mm: Avoid flushing the TLBs when mapping are inserted Julien Grall
2021-05-11 22:51   ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 06/15] xen/arm: mm: Don't open-code Xen PT update in remove_early_mappings() Julien Grall
2021-05-11 22:58   ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 07/15] xen/arm: mm: Re-implement early_fdt_map() using map_pages_to_xen() Julien Grall
2021-05-12 21:41   ` Stefano Stabellini
2021-05-12 22:18     ` Julien Grall
2021-04-25 20:13 ` [PATCH RFCv2 08/15] xen/arm32: mm: Check if the virtual address is shared before updating it Julien Grall
2021-05-12 22:00   ` Stefano Stabellini
2021-05-12 22:23     ` Julien Grall
2021-05-13 22:32       ` Stefano Stabellini
2021-05-13 22:59         ` Julien Grall
2021-05-14  1:04           ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 09/15] xen/arm32: mm: Re-implement setup_xenheap_mappings() using map_pages_to_xen() Julien Grall
2021-05-12 22:07   ` Stefano Stabellini
2021-05-13 17:55     ` Julien Grall
2021-04-25 20:13 ` [PATCH RFCv2 10/15] xen/arm: mm: Allocate xen page tables in domheap rather than xenheap Julien Grall
2021-05-12 22:44   ` Stefano Stabellini
2021-05-13 18:09     ` Julien Grall
2021-05-13 22:27       ` Stefano Stabellini
2021-05-15  8:48         ` Julien Grall [this message]
2021-05-18  0:37           ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 11/15] xen/arm: mm: Allow page-table allocation from the boot allocator Julien Grall
2021-05-13 22:58   ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 12/15] xen/arm: add Persistent Map (PMAP) infrastructure Julien Grall
2021-04-26  9:41   ` Xia, Hongyan
2021-07-03 17:57     ` Julien Grall
2021-04-28 21:47   ` Wei Liu
2021-05-14 23:25   ` Stefano Stabellini
2021-05-15  8:54     ` Julien Grall
2021-04-25 20:13 ` [PATCH RFCv2 13/15] xen/arm: mm: Use the PMAP helpers in xen_{,un}map_table() Julien Grall
2021-05-14 23:35   ` Stefano Stabellini
2021-05-15  9:03     ` Julien Grall
2021-04-25 20:13 ` [PATCH RFCv2 14/15] xen/arm: mm: Rework setup_xenheap_mappings() Julien Grall
2021-05-14 23:51   ` Stefano Stabellini
2021-05-15  9:21     ` Julien Grall
2021-05-18  0:50       ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 15/15] xen/arm: mm: Re-implement setup_frame_table_mappings() with map_pages_to_xen() Julien Grall
2021-05-15  0:02   ` Stefano Stabellini
2021-05-15  9:25     ` Julien Grall
2021-05-18  0:51       ` Stefano Stabellini

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