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

["Xia, Hongyan" <hongyax@xxxxxxxxxx>]

Not commenting on the patch, but I had exactly the same problem when
removing the direct map in x86. map_domain_page has to be usable
without the direct map and even before alloc_boot_pages can be used (so
that I can map the bootmem_regions_list, although I have made it
statically allocated now).

The direct map teardown series in the end introduces a persistent
mapping infrastructure to map page tables which occupies a few fixmap
entries, and avoid set_fixmap but instead modify the entries directly
to avoid invocation loops. The result is that map_xen_pagetable and
map_domain_page is usable from the very beginning of start_xen.

Hongyan

On Tue, 2019-09-17 at 17:02 +0100, 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>
> ---
>  xen/arch/arm/mm.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index e1cdeaaf2f..da6303a8fd 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -950,11 +950,31 @@ static int create_xen_table(lpae_t *entry)
>  
>  static lpae_t *xen_map_table(mfn_t mfn)
>  {
> +    /*
> +     * We may require to map the page table before map_domain_page()
> is
> +     * useable. The requirements here is it must be useable as soon
> as
> +     * page-tables are allocated dynamically via alloc_boot_pages().
> +     */
> +    if ( system_state == SYS_STATE_early_boot )
> +    {
> +        vaddr_t va = mfn_to_maddr(mfn) - phys_offset;
> +
> +        if ( is_kernel(va) )
> +            return (lpae_t *)va;
> +    }
> +
>      return map_domain_page(mfn);
>  }
>  
>  static void xen_unmap_table(const lpae_t *table)
>  {
> +    /*
> +     * During early boot, xen_map_table() will not use
> map_domain_page()
> +     * for page-tables residing in Xen binary. So skip the unmap
> part.
> +     */
> +    if ( system_state == SYS_STATE_early_boot && is_kernel(table) )
> +        return;
> +
>      unmap_domain_page(table);
>  }
>  
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