Re: [Xen-devel] [RFC 07/16] xen/arm: p2m: Introduce p2m_is_valid and use it

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 8 Oct 2018, Julien Grall wrote:
> The LPAE format allows to store information in an entry even with the
> valid bit unset. In a follow-up patch, we will take advantage of this
> feature to re-purpose the valid bit for generating a translation fault
> even if an entry contains valid information.
> 
> So we need a different way to know whether an entry contains valid
> information. It is possible to use the information hold in the p2m_type
> to know for that purpose. Indeed all entries containing valid
> information will have a valid p2m type (i.e p2m_type != p2m_invalid).
> 
> This patch introduces a new helper p2m_is_valid, which implements that
> idea, and replace most of lpae_is_valid call with the new helper. The ones
> remaining are for TLBs handling and entries accounting.
> 
> With the renaming there are 2 others changes required:
>     - Generate table entry with a valid p2m type
>     - Detect new mapping for proper stats accounting
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> ---
>  xen/arch/arm/p2m.c | 34 +++++++++++++++++++++++-----------
>  1 file changed, 23 insertions(+), 11 deletions(-)
> 
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 6c76298ebc..2a1e7e9be2 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -220,17 +220,26 @@ static p2m_access_t p2m_mem_access_radix_get(struct 
> p2m_domain *p2m, gfn_t gfn)
>  }
>  
>  /*
> + * In the case of the P2M, the valid bit is used for other purpose. Use
> + * the type to check whether an entry is valid.
> + */
> +static inline bool p2m_is_valid(lpae_t pte)
> +{
> +    return pte.p2m.type != p2m_invalid;
> +}
> +
> +/*
>   * lpae_is_* helpers don't check whether the valid bit is set in the
>   * PTE. Provide our own overlay to check the valid bit.
>   */
>  static inline bool p2m_is_mapping(lpae_t pte, unsigned int level)
>  {
> -    return lpae_is_valid(pte) && lpae_is_mapping(pte, level);
> +    return p2m_is_valid(pte) && lpae_is_mapping(pte, level);
>  }
>  
>  static inline bool p2m_is_superpage(lpae_t pte, unsigned int level)
>  {
> -    return lpae_is_valid(pte) && lpae_is_superpage(pte, level);
> +    return p2m_is_valid(pte) && lpae_is_superpage(pte, level);
>  }
>  
>  #define GUEST_TABLE_MAP_FAILED 0
> @@ -264,7 +273,7 @@ static int p2m_next_level(struct p2m_domain *p2m, bool 
> read_only,
>  
>      entry = *table + offset;
>  
> -    if ( !lpae_is_valid(*entry) )
> +    if ( !p2m_is_valid(*entry) )
>      {
>          if ( read_only )
>              return GUEST_TABLE_MAP_FAILED;
> @@ -356,7 +365,7 @@ mfn_t p2m_get_entry(struct p2m_domain *p2m, gfn_t gfn,
>  
>      entry = table[offsets[level]];
>  
> -    if ( lpae_is_valid(entry) )
> +    if ( p2m_is_valid(entry) )
>      {
>          *t = entry.p2m.type;
>  
> @@ -544,8 +553,11 @@ static lpae_t page_to_p2m_table(struct page_info *page)
>      /*
>       * The access value does not matter because the hardware will ignore
>       * the permission fields for table entry.
> +     *
> +     * We use p2m_ram_rw so the entry has a valid type. This is important
> +     * for p2m_is_valid() to return valid on table entries.
>       */
> -    return mfn_to_p2m_entry(page_to_mfn(page), p2m_invalid, p2m_access_rwx);
> +    return mfn_to_p2m_entry(page_to_mfn(page), p2m_ram_rw, p2m_access_rwx);
>  }
>  
>  static inline void p2m_write_pte(lpae_t *p, lpae_t pte, bool clean_pte)
> @@ -569,7 +581,7 @@ static int p2m_create_table(struct p2m_domain *p2m, 
> lpae_t *entry)
>      struct page_info *page;
>      lpae_t *p;
>  
> -    ASSERT(!lpae_is_valid(*entry));
> +    ASSERT(!p2m_is_valid(*entry));
>  
>      page = alloc_domheap_page(NULL, 0);
>      if ( page == NULL )
> @@ -626,7 +638,7 @@ static int p2m_mem_access_radix_set(struct p2m_domain 
> *p2m, gfn_t gfn,
>   */
>  static void p2m_put_l3_page(const lpae_t pte)
>  {
> -    ASSERT(lpae_is_valid(pte));
> +    ASSERT(p2m_is_valid(pte));
>  
>      /*
>       * TODO: Handle other p2m types
> @@ -654,11 +666,11 @@ static void p2m_free_entry(struct p2m_domain *p2m,
>      struct page_info *pg;
>  
>      /* Nothing to do if the entry is invalid. */
> -    if ( !lpae_is_valid(entry) )
> +    if ( !p2m_is_valid(entry) )
>          return;
>  
>      /* Nothing to do but updating the stats if the entry is a super-page. */
> -    if ( p2m_is_superpage(entry, level) )
> +    if ( level == 3 && entry.p2m.table )

Why?


>      {
>          p2m->stats.mappings[level]--;
>          return;
> @@ -951,7 +963,7 @@ static int __p2m_set_entry(struct p2m_domain *p2m,
>              else
>                  p2m->need_flush = true;
>          }
> -        else /* new mapping */
> +        else if ( !p2m_is_valid(orig_pte) ) /* new mapping */

There are a couple of lpae_is_valid checks just above this line that you
missed, why haven't you changed them?

If you have a good reason, please explain in a comment and/or commit
message.

>              p2m->stats.mappings[level]++;
>  
>          p2m_write_pte(entry, pte, p2m->clean_pte);
> @@ -965,7 +977,7 @@ static int __p2m_set_entry(struct p2m_domain *p2m,
>       * Free the entry only if the original pte was valid and the base
>       * is different (to avoid freeing when permission is changed).
>       */
> -    if ( lpae_is_valid(orig_pte) &&
> +    if ( p2m_is_valid(orig_pte) &&
>           !mfn_eq(lpae_get_mfn(*entry), lpae_get_mfn(orig_pte)) )
>          p2m_free_entry(p2m, orig_pte, level);
>  

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