Re: [Xen-devel] [PATCH MM-PART3 v2 11/12] xen/arm: mm: Don't open-code Xen PT update in {set, clear}_fixmap()

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 14 May 2019, Julien Grall wrote:
> {set, clear}_fixmap() are currently open-coding update to the Xen
> page-tables. This can be avoided by using the generic helpers
> map_pages_to_xen() and destroy_xen_mappings().
> 
> Both function are not meant to fail for fixmap, hence the BUG_ON()
> checking the return.

BUG_ON crashes the hypervisor even in non-DEBUG builds. Would an ASSERT
be a better choice?

The changes in this patch checks out OK.


> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> Reviewed-by: Andrii Anisov <andrii_anisov@xxxxxxxx>
> 
> ---
>     Changes in v2:
>         - Add Andrii's reviewed-by
> ---
>  xen/arch/arm/mm.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index 9a40754f44..23ca61e8f0 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -348,19 +348,19 @@ static inline lpae_t mfn_to_xen_entry(mfn_t mfn, 
> unsigned attr)
>  /* Map a 4k page in a fixmap entry */
>  void set_fixmap(unsigned map, mfn_t mfn, unsigned int flags)
>  {
> -    lpae_t pte = mfn_to_xen_entry(mfn, PAGE_AI_MASK(flags));
> -    pte.pt.table = 1; /* 4k mappings always have this bit set */
> -    pte.pt.xn = 1;
> -    write_pte(xen_fixmap + third_table_offset(FIXMAP_ADDR(map)), pte);
> -    flush_xen_tlb_range_va(FIXMAP_ADDR(map), PAGE_SIZE);
> +    int res;
> +
> +    res = map_pages_to_xen(FIXMAP_ADDR(map), mfn, 1, flags);
> +    BUG_ON(res != 0);
>  }
>  
>  /* Remove a mapping from a fixmap entry */
>  void clear_fixmap(unsigned map)
>  {
> -    lpae_t pte = {0};
> -    write_pte(xen_fixmap + third_table_offset(FIXMAP_ADDR(map)), pte);
> -    flush_xen_tlb_range_va(FIXMAP_ADDR(map), PAGE_SIZE);
> +    int res;
> +
> +    res = destroy_xen_mappings(FIXMAP_ADDR(map), FIXMAP_ADDR(map) + 
> PAGE_SIZE);
> +    BUG_ON(res != 0);
>  }
>  
>  /* Create Xen's mappings of memory.
> -- 
> 2.11.0
> 

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