[Xen-devel] dom/xen heap and boot allocator (WAS Re: [xen-unstable-smoke test] 141333: regressions - FAIL)

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 9/15/19 3:09 PM, osstest service owner wrote:
> flight 141333 xen-unstable-smoke real [real]
> http://logs.test-lab.xenproject.org/osstest/logs/141333/
>
> Regressions :-(
>
> Tests which did not succeed and are blocking,
> including tests which could not be run:
>   test-armhf-armhf-xl           7 xen-boot                 fail REGR. vs. 141253
Osstest does not provide the stack trace as the crash happen before the 
console is setup, but I managed to reproduce it locally:
(XEN) Assertion 'is_xen_heap_mfn(maddr_to_mfn(ma))' failed at 
/home/julieng/works/xen/xen/include/asm/mm.h:250
(XEN) ----[ Xen-4.13-unstable  arm32  debug=y   Not tainted ]----

[...]

(XEN) Xen call trace:
(XEN)    [<002992c0>] page_alloc.c#bootmem_region_add+0xf8/0x17c (PC)
(XEN)    [<002995ac>] init_boot_pages+0x8c/0x1a0 (LR)
(XEN)    [<002995ac>] init_boot_pages+0x8c/0x1a0
(XEN)    [<002acc2c>] dt_unreserved_regions+0x268/0x284
(XEN)    [<002ad8e0>] start_xen+0x5c8/0xe90
(XEN)    [<00200098>] arm32/head.o#primary_switched+0x4/0x10

This is happening because of commit 6e3e771203 "xen/arm: setup: Relocate 
the Device-Tree later on in the boot". Since this patch, none of xenheap 
memory is given to the boot allocator.
The boot allocator is bootstrapping itself and re-use a page from the 
first added region. If this region is not a xenheap region, then it will 
crash when calling mfn_to_virt() or later on because the virtual address 
is not mapped in memory.
The description of the boot allocator in page_alloc.c leads to think 
that only domheap memory may be given to the boot allocator.
Furthermore, as the boot allocator may have domheap page, it means that 
calling mfn_to_virt(mfn_x(alloc_boot_pages(...)) may not work when 
CONFIG_SEPARATE_XENHEAP=y.
It feels to me that imposing to give a xenheap page to the boot 
allocator is quite ugly. As the boot allocator will be used in most of 
the case, statically allocating bootmem_region_list maybe the best. Any 
thoughts?
Regarding alloc_boot_pages(), I am a bit unsure how to proceed here. 
Maybe some documentation?
Cheers,

--
Julien Grall

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