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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 30 Oct 2018, Julien Grall wrote:
> Hi,
> 
> On 30/10/2018 00:21, Stefano Stabellini wrote:
> > 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?
> 
> Because p2m_is_superpage(...) contains p2m_is_valid(). So we would test twice
> the validity of the p2m.
> 
> But I guess this is not a big deal, so I can remove it.
> 
> > 
> > 
> > >       {
> > >           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.
> 
> This is already explained in the commit message:
> 
> "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."
> 
> I believe that the code has enough existing comment to understand why
> lpae_is_valid(...) should be kept. You deal with hardware update and hence you
> should use the valid bit in the LPAE table. This will tell you whether you
> need to flush the TLBs.

I checked and it is like you wrote.


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