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From: Stefano Stabellini <sstabellini@kernel.org>
To: Julien Grall <julien@xen.org>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	 xen-devel@lists.xenproject.org, Wei.Chen@arm.com,
	Henry.Wang@arm.com,  Penny.Zheng@arm.com,
	Bertrand.Marquis@arm.com,  Julien Grall <julien.grall@arm.com>,
	 Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>,
	 Julien Grall <jgrall@amazon.com>
Subject: Re: [PATCH RFCv2 14/15] xen/arm: mm: Rework setup_xenheap_mappings()
Date: Mon, 17 May 2021 17:50:30 -0700 (PDT)	[thread overview]
Message-ID: <alpine.DEB.2.21.2105171738320.14426@sstabellini-ThinkPad-T480s> (raw)
In-Reply-To: <8cda6d2d-7f3c-fef6-c534-2fadabed1bad@xen.org>

On Sat, 15 May 2021, Julien Grall wrote:
> Hi,
> 
> On 15/05/2021 00:51, Stefano Stabellini wrote:
> > On Sun, 25 Apr 2021, Julien Grall wrote:
> > > From: Julien Grall <julien.grall@arm.com>
> > > 
> > > A few issues have been reported with setup_xenheap_mappings() over the
> > > last couple of years. The main ones are:
> > >      - It will break on platform supporting more than 512GB of RAM
> > >        because the memory allocated by the boot allocator is not yet
> > >        mapped.
> > >      - Aligning all the regions to 1GB may lead to unexpected result
> > >        because we may alias non-cacheable region (such as device or
> > > reserved
> > >        regions).
> > > 
> > > map_pages_to_xen() was recently reworked to allow superpage mappings and
> > > deal with the use of page-tables before they are mapped.
> > > 
> > > Most of the code in setup_xenheap_mappings() is now replaced with a
> > > single call to map_pages_to_xen().
> > > 
> > > This also require to re-order the steps setup_mm() so the regions are
> > > given to the boot allocator first and then we setup the xenheap
> > > mappings.
> > 
> > I know this is paranoia but doesn't this introduce a requirement on the
> > size of the first bank in bootinfo.mem.bank[] ?
> > 
> > It should be at least 8KB?
> 
> AFAIK, the current requirement is 4KB because of the 1GB mapping. Long term,
> it would be 8KB.
> 
> > 
> > I know it is unlikely but it is theoretically possible to have a first
> > bank of just 1KB.
> 
> All the page allocators are working at the page granularity level. I am not
> entirely sure whether the current Xen would ignore the region or break.
> 
> Note that setup_xenheap_mappings() is taking a base MFN and a number of pages
> to map. So this doesn't look to be a new problem here.

Yeah... the example of the first bank being 1KB is wrong because, like
you wrote, it wouldn't work before your patches either, and probably it
will never work.

Maybe a better example is a first bank of 4KB exactly.


> For the 8KB requirement, we can look at first all the pages to the boot
> allocator and then do the mapping.
> 
> > 
> > I think we should write the requirement down with an in-code comment?
> > Or better maybe we should write a message about it in the panic below?
> 
> I can write an in-code comment but I think writing down in the panic() would
> be wrong because there is no promise this is going to be the only reason it
> fails (imagine there is a bug in the code...).

Maybe it is sufficient to print out the error code (EINVAL / ENOMEM etc)
in the panic. If you see -12 you know what to look for.

Looking into it I realize that we are not returning -ENOMEM in case of
alloc_boot_pages failures in create_xen_table. It looks like we would
hit a BUG() in the implementation of alloc_boot_pages. Maybe that's good
enough as is then.


  reply	other threads:[~2021-05-18  0:50 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
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 [this message]
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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