Re: [PATCH RFCv2 02/15] xen/arm: lpae: Use the generic helpers to defined the Xen PT helpers

[Stefano Stabellini <sstabellini@kernel.org>]

On Sun, 25 Apr 2021, Julien Grall wrote:
> From: Julien Grall <jgrall@amazon.com>
> 
> Currently, Xen PT helpers are only working with 4KB page granularity
> and open-code the generic helpers. To allow more flexibility, we can
> re-use the generic helpers and pass Xen's page granularity
> (PAGE_SHIFT).
> 
> As Xen PT helpers are used in both C and assembly, we need to move
> the generic helpers definition outside of the !__ASSEMBLY__ section.
> 
> Note the aliases for each level are still kept for the time being so we
> can avoid a massive patch to change all the callers.
> 
> Signed-off-by: Julien Grall <jgrall@amazon.com>

The patch is OK as is. I have a couple of suggestions for improvement
below. If you feel like making them, good, otherwise I am also OK if you
don't want to change anything.


> ---
>     Changes in v2:
>         - New patch
> ---
>  xen/include/asm-arm/lpae.h | 71 +++++++++++++++++++++-----------------
>  1 file changed, 40 insertions(+), 31 deletions(-)
> 
> diff --git a/xen/include/asm-arm/lpae.h b/xen/include/asm-arm/lpae.h
> index 4fb9a40a4ca9..310f5225e056 100644
> --- a/xen/include/asm-arm/lpae.h
> +++ b/xen/include/asm-arm/lpae.h
> @@ -159,6 +159,17 @@ static inline bool lpae_is_superpage(lpae_t pte, unsigned int level)
>  #define lpae_get_mfn(pte)    (_mfn((pte).walk.base))
>  #define lpae_set_mfn(pte, mfn)  ((pte).walk.base = mfn_x(mfn))
>  
> +/* Generate an array @var containing the offset for each level from @addr */
> +#define DECLARE_OFFSETS(var, addr)          \
> +    const unsigned int var[4] = {           \
> +        zeroeth_table_offset(addr),         \
> +        first_table_offset(addr),           \
> +        second_table_offset(addr),          \
> +        third_table_offset(addr)            \
> +    }
> +
> +#endif /* __ASSEMBLY__ */
> +
>  /*
>   * AArch64 supports pages with different sizes (4K, 16K, and 64K).
>   * Provide a set of generic helpers that will compute various
> @@ -190,17 +201,6 @@ static inline bool lpae_is_superpage(lpae_t pte, unsigned int level)
>  #define LPAE_TABLE_INDEX_GS(gs, lvl, addr)   \
>      (((addr) >> LEVEL_SHIFT_GS(gs, lvl)) & LPAE_ENTRY_MASK_GS(gs))
>  
> -/* Generate an array @var containing the offset for each level from @addr */
> -#define DECLARE_OFFSETS(var, addr)          \
> -    const unsigned int var[4] = {           \
> -        zeroeth_table_offset(addr),         \
> -        first_table_offset(addr),           \
> -        second_table_offset(addr),          \
> -        third_table_offset(addr)            \
> -    }
> -
> -#endif /* __ASSEMBLY__ */
> -
>  /*
>   * These numbers add up to a 48-bit input address space.
>   *
> @@ -211,26 +211,35 @@ static inline bool lpae_is_superpage(lpae_t pte, unsigned int level)
>   * therefore 39-bits are sufficient.
>   */
>  
> -#define LPAE_SHIFT      9
> -#define LPAE_ENTRIES    (_AC(1,U) << LPAE_SHIFT)
> -#define LPAE_ENTRY_MASK (LPAE_ENTRIES - 1)
> -
> -#define THIRD_SHIFT    (PAGE_SHIFT)
> -#define THIRD_ORDER    (THIRD_SHIFT - PAGE_SHIFT)
> -#define THIRD_SIZE     (_AT(paddr_t, 1) << THIRD_SHIFT)
> -#define THIRD_MASK     (~(THIRD_SIZE - 1))
> -#define SECOND_SHIFT   (THIRD_SHIFT + LPAE_SHIFT)
> -#define SECOND_ORDER   (SECOND_SHIFT - PAGE_SHIFT)
> -#define SECOND_SIZE    (_AT(paddr_t, 1) << SECOND_SHIFT)
> -#define SECOND_MASK    (~(SECOND_SIZE - 1))
> -#define FIRST_SHIFT    (SECOND_SHIFT + LPAE_SHIFT)
> -#define FIRST_ORDER    (FIRST_SHIFT - PAGE_SHIFT)
> -#define FIRST_SIZE     (_AT(paddr_t, 1) << FIRST_SHIFT)
> -#define FIRST_MASK     (~(FIRST_SIZE - 1))
> -#define ZEROETH_SHIFT  (FIRST_SHIFT + LPAE_SHIFT)
> -#define ZEROETH_ORDER  (ZEROETH_SHIFT - PAGE_SHIFT)
> -#define ZEROETH_SIZE   (_AT(paddr_t, 1) << ZEROETH_SHIFT)
> -#define ZEROETH_MASK   (~(ZEROETH_SIZE - 1))

Should we add a one-line in-code comment saying that the definitions
below are for 4KB pages? It is not immediately obvious any longer.


> +#define LPAE_SHIFT          LPAE_SHIFT_GS(PAGE_SHIFT)
> +#define LPAE_ENTRIES        LPAE_ENTRIES_GS(PAGE_SHIFT)
> +#define LPAE_ENTRY_MASK     LPAE_ENTRY_MASK_GS(PAGE_SHIFT)
>
> +#define LEVEL_SHIFT(lvl)    LEVEL_SHIFT_GS(PAGE_SHIFT, lvl)
> +#define LEVEL_ORDER(lvl)    LEVEL_ORDER_GS(PAGE_SHIFT, lvl)
> +#define LEVEL_SIZE(lvl)     LEVEL_SIZE_GS(PAGE_SHIFT, lvl)
> +#define LEVEL_MASK(lvl)     (~(LEVEL_SIZE(lvl) - 1))

I would avoid adding these 4 macros. It would be OK if they were just
used within this file but lpae.h is a header: they could end up be used
anywhere in the xen/ code and they have a very generic name. My
suggestion would be to skip them and just do:

#define THIRD_SHIFT         LEVEL_SHIFT_GS(PAGE_SHIFT, 3)

etc.


> +/* Convenience aliases */
> +#define THIRD_SHIFT         LEVEL_SHIFT(3)
> +#define THIRD_ORDER         LEVEL_ORDER(3)
> +#define THIRD_SIZE          LEVEL_SIZE(3)
> +#define THIRD_MASK          LEVEL_MASK(3)
> +
> +#define SECOND_SHIFT        LEVEL_SHIFT(2)
> +#define SECOND_ORDER        LEVEL_ORDER(2)
> +#define SECOND_SIZE         LEVEL_SIZE(2)
> +#define SECOND_MASK         LEVEL_MASK(2)
> +
> +#define FIRST_SHIFT         LEVEL_SHIFT(1)
> +#define FIRST_ORDER         LEVEL_ORDER(1)
> +#define FIRST_SIZE          LEVEL_SIZE(1)
> +#define FIRST_MASK          LEVEL_MASK(1)
> +
> +#define ZEROETH_SHIFT       LEVEL_SHIFT(0)
> +#define ZEROETH_ORDER       LEVEL_ORDER(0)
> +#define ZEROETH_SIZE        LEVEL_SIZE(0)
> +#define ZEROETH_MASK        LEVEL_MASK(0)
>  
>  /* Calculate the offsets into the pagetables for a given VA */
>  #define zeroeth_linear_offset(va) ((va) >> ZEROETH_SHIFT)
> -- 
> 2.17.1
>