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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 10 Oct 2019, 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>
> 
> ---
>     Changes in v2:
>         - Add RaB Juergen
>         - Rework the check to avoid truncation
> ---
>  xen/arch/arm/mm.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index be23acfe26..a6637ce347 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -964,11 +964,40 @@ 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().
> +     *
> +     * We need to do the check on physical address rather than virtual
> +     * address to avoid truncation on Arm32. Therefore is_kernel() cannot
> +     * be used.
> +     */
> +    if ( system_state == SYS_STATE_early_boot )
> +    {
> +        const mfn_t mstart = virt_to_mfn(_start);
> +        const mfn_t mend = virt_to_mfn(_end);
> +
> +        if ( (mfn_x(mstart) <= mfn_x(mfn)) && (mfn_x(mfn) < mfn_x(mend)) )
> +        {

The patch is good. Actually I noticed that we already have
is_xen_fixed_mfn(), looks like it is made for this. I think we should
use it here, except that is_xen_fixed_mfn has:

     (mfn_to_maddr(mfn) <= virt_to_maddr(&_end)))

I think it should actually be:

     (mfn_to_maddr(mfn) < virt_to_maddr(&_end)))

Maybe we could fix that at the same time in one patch? However, it is
the same definition as on x86, so I don't know what is going on there.


> +            vaddr_t offset = mfn_to_maddr(mfn) - mfn_to_maddr(mstart);
> +            return (lpae_t *)(XEN_VIRT_START + offset);

I know this is safe in this case thanks to the `if' above, so there are
no risks. But in general it is not a great idea to have a hidden 64-bit
to 32-bit cast (also it is not MISRA compliant.) I would add an explicit
cast:

  vaddr_t offset = (vaddr_t)(mfn_to_maddr(mfn) - mfn_to_maddr(mstart));

I am not going to insist on this though.


> +        }
> +    }
> +
>      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);
>  }
>  
> -- 
> 2.11.0
> 

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