Re: [Xen-devel] [PATCH MM-PART3 v2 09/12] xen/arm: mm: Use {, un}map_domain_page() to map/unmap Xen page-tables

[Stefano Stabellini <sstabellini@kernel.org>]

On Tue, 14 May 2019, Julien Grall wrote:
> Currently, the virtual address of the 3rd level page-tables is obtained
> using mfn_to_virt().
> 
> On Arm32, mfn_to_virt can only work on xenheap page. While in practice
> all the page-tables updated will reside in xenheap, in practive the
                                                        ^ in theory ?


> page-tables covering Xen memory (e.g xen_mapping) is part of Xen binary.
> 
> Furthermore, a follow-up change will update xen_pt_update_entry() to
> walk all the levels and therefore be more generic. Some of the
> page-tables will also part of Xen memory and therefore will not be
> reachable using mfn_to_virt().
> 
> The easiest way to reach those pages is to use {, un}map_domain_page().
> While on arm32 this means an extra mapping in the normal cases, this is not
> very important as xen page-tables are not updated often.
> 
> In order to allow future change in the way Xen page-tables are mapped,
> two new helpers are introduced to map/unmap the page-tables.
> 
> Signed-off-by: Julien Grall <julien.grall@arm.com>
> Reviewed-by: Andrii Anisov <andrii_anisov@epam.com>

aside from the typo above:

Acked-by: Stefano Stabellini <sstabellini@kernel.org>


> ---
> 
>     Changes in v2:
>         - Add Andrii's reviewed-by
> ---
>  xen/arch/arm/mm.c | 26 ++++++++++++++++++++++----
>  1 file changed, 22 insertions(+), 4 deletions(-)
> 
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index 651e296041..f5979f549b 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -974,6 +974,16 @@ static int create_xen_table(lpae_t *entry)
>      return 0;
>  }
>  
> +static lpae_t *xen_map_table(mfn_t mfn)
> +{
> +    return map_domain_page(mfn);
> +}
> +
> +static void xen_unmap_table(const lpae_t *table)
> +{
> +    unmap_domain_page(table);
> +}
> +
>  /* Sanity check of the entry */
>  static bool xen_pt_check_entry(lpae_t entry, mfn_t mfn, unsigned int flags)
>  {
> @@ -1036,6 +1046,7 @@ static bool xen_pt_check_entry(lpae_t entry, mfn_t mfn, unsigned int flags)
>  static int xen_pt_update_entry(unsigned long addr, mfn_t mfn,
>                                 unsigned int flags)
>  {
> +    int rc;
>      lpae_t pte, *entry;
>      lpae_t *third = NULL;
>  
> @@ -1054,15 +1065,17 @@ static int xen_pt_update_entry(unsigned long addr, mfn_t mfn,
>  
>      BUG_ON(!lpae_is_valid(*entry));
>  
> -    third = mfn_to_virt(lpae_get_mfn(*entry));
> +    third = xen_map_table(lpae_get_mfn(*entry));
>      entry = &third[third_table_offset(addr)];
>  
> +    rc = -EINVAL;
>      if ( !xen_pt_check_entry(*entry, mfn, flags) )
> -        return -EINVAL;
> +        goto out;
>  
>      /* If we are only populating page-table, then we are done. */
> +    rc = 0;
>      if ( flags & _PAGE_POPULATE )
> -        return 0;
> +        goto out;
>  
>      /* We are removing the page */
>      if ( !(flags & _PAGE_PRESENT) )
> @@ -1087,7 +1100,12 @@ static int xen_pt_update_entry(unsigned long addr, mfn_t mfn,
>  
>      write_pte(entry, pte);
>  
> -    return 0;
> +    rc = 0;
> +
> +out:
> +    xen_unmap_table(third);
> +
> +    return rc;
>  }
>  
>  static DEFINE_SPINLOCK(xen_pt_lock);
> -- 
> 2.11.0
> 

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