Re: [Xen-devel] [[PATCH for-4.13]] xen/arm: mm: Allow generic xen page-tables helpers to be called early

[Jürgen Groß <jgross@xxxxxxxx>]

On 25.09.19 17:23, Julien Grall wrote:
> Hi,
>
> I am CCing Juergen to mark this as a blocker for 4.13. Without this 
> patch, Xen cannot be booted using GRUB.
> Cheers,
>
> On 17/09/2019 17:02, Julien Grall wrote:
> > The current implementations of xen_{map, unmap}_table() expect
> > {map, unmap}_domain_page() to be usable. Those helpers are used to
> > map/unmap page tables while update Xen page-tables.
> >
> > Since commit 022387ee1a "xen/arm: mm: Don't open-code Xen PT update in
> > {set, clear}_fixmap()", setup_fixmap() will make use of the helpers
> > mentioned above. When booting Xen using GRUB, setup_fixmap() may be used
> > before map_domain_page() can be called. This will result to data abort:
> >
> > (XEN) Data Abort Trap. Syndrome=0x5
> > (XEN) CPU0: Unexpected Trap: Data Abort
> >
> > [...]
> >
> > (XEN) Xen call trace:
> > (XEN)    [<000000000025ab6c>] mm.c#xen_pt_update+0x2b4/0x59c (PC)
> > (XEN)    [<000000000025ab20>] mm.c#xen_pt_update+0x268/0x59c (LR)
> > (XEN)    [<000000000025ae70>] set_fixmap+0x1c/0x2c
> > (XEN)    [<00000000002a9c98>] copy_from_paddr+0x7c/0xdc
> > (XEN)    [<00000000002a4ae0>] has_xsm_magic+0x18/0x34
> > (XEN)    [<00000000002a5b5c>] bootfdt.c#early_scan_node+0x398/0x560
> > (XEN)    [<00000000002a5de0>] device_tree_for_each_node+0xbc/0x144
> > (XEN)    [<00000000002a5ed4>] boot_fdt_info+0x6c/0x260
> > (XEN)    [<00000000002ac0d0>] start_xen+0x108/0xc74
> > (XEN)    [<000000000020044c>] arm64/head.o#paging+0x60/0x88
> >
> > During early boot, the page tables are either statically allocated in
> > Xen binary or allocated via alloc_boot_pages().
> >
> > For statically allocated page-tables, they will already be mapped as
> > part of Xen binary. So we can easily find the virtual address.
> >
> > For dynamically allocated page-tables, we need to rely
> > map_domain_page() to be functionally working.
> >
> > For arm32, the call will be usable much before page can be dynamically
> > allocated (see setup_pagetables()). For arm64, the call will be usable
> > after setup_xenheap_mappings().
> >
> > In both cases, memory are given to the boot allocator afterwards. So we
> > can rely on map_domain_page() for mapping page tables allocated
> > dynamically.
> >
> > The helpers xen_{map, unmap}_table() are now updated to take into
> > account the case where page-tables are part of Xen binary.
> >
> > Fixes: 022387ee1a ('xen/arm: mm: Don't open-code Xen PT update in 
> > {set, clear}_fixmap()')
> > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
Release-acked-by: Juergen Gross <jgross@xxxxxxxx>


Juergen

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