Re: [Xen-devel] [PATCH 11/15] xen/arm: Allow lpae_is_{table, mapping} helpers to work on invalid entry

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 08/14/2018 10:33 PM, Stefano Stabellini wrote:
> On Mon, 16 Jul 2018, Julien Grall wrote:
> > Currently, lpae_is_{table, mapping} helpers will always return false on
> > entry with the valid bit unset. However, it would be useful to have them
>    ^entries
>
> > operating on any entry. For instance to store information in advance but
> > still request a fault.
> >
> > With that change, the p2m is now providing an overlay for *_is_{table,
> > mapping} that will check the valid bit of the entry.
> >
> > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> >
> > ---
> >   xen/arch/arm/guest_walk.c  |  2 +-
> >   xen/arch/arm/mm.c          |  2 +-
> >   xen/arch/arm/p2m.c         | 22 ++++++++++++++++++----
> >   xen/include/asm-arm/lpae.h | 11 +++++++----
> >   4 files changed, 27 insertions(+), 10 deletions(-)
> >
> > diff --git a/xen/arch/arm/guest_walk.c b/xen/arch/arm/guest_walk.c
> > index e3e21bdad3..4a1b4cf2c8 100644
> > --- a/xen/arch/arm/guest_walk.c
> > +++ b/xen/arch/arm/guest_walk.c
> > @@ -566,7 +566,7 @@ static int guest_walk_ld(const struct vcpu *v,
> >        * PTE is invalid or holds a reserved entry (PTE<1:0> == x0)) or if the 
> > PTE
> >        * maps a memory block at level 3 (PTE<1:0> == 01).
> >        */
> > -    if ( !lpae_is_mapping(pte, level) )
> > +    if ( !lpae_is_valid(pte) || !lpae_is_mapping(pte, level) )
> >           return -EFAULT;
> >   
> >       /* Make sure that the lower bits of the PTE's base address are zero. */
> > diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> > index e3dafe5fd7..52e57fef2f 100644
> > --- a/xen/arch/arm/mm.c
> > +++ b/xen/arch/arm/mm.c
> > @@ -996,7 +996,7 @@ static int create_xen_entries(enum xenmap_operation op,
> >       for(; addr < addr_end; addr += PAGE_SIZE, mfn = mfn_add(mfn, 1))
> >       {
> >           entry = &xen_second[second_linear_offset(addr)];
> > -        if ( !lpae_is_table(*entry, 2) )
> > +        if ( !lpae_is_valid(*entry) || !lpae_is_table(*entry, 2) )
> >           {
> >               rc = create_xen_table(entry);
> >               if ( rc < 0 ) {
> > diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> > index ec3fdcb554..07925a1be4 100644
> > --- a/xen/arch/arm/p2m.c
> > +++ b/xen/arch/arm/p2m.c
> > @@ -219,6 +219,20 @@ static p2m_access_t p2m_mem_access_radix_get(struct 
> > p2m_domain *p2m, gfn_t gfn)
> >           return radix_tree_ptr_to_int(ptr);
> >   }
> >   
> > +/*
> > + * 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);
> > +}
> > +
> > +static inline bool p2m_is_superpage(lpae_t pte, unsigned int level)
> > +{
> > +    return lpae_is_valid(pte) && lpae_is_superpage(pte, level);
> > +}
> I like the idea, but it would be clearer to me if they were called
> lpae_is_valid_mapping and lpae_is_valid_superpage respectively. What do
> you think?
>  > Also, we might as well move them to lpae.h and use them from mm.c and
> guest_walk.c as appropriate?
lpae.h is not suitable because as I said in the commit message each 
page-table (stage-1, stage-2) may have a different view of what "valid" 
means.
At the moment, that view is the same but it is not going to stay long 
like that. Hence the reason of prefixing with p2m_. They are specific to 
the p2m. This is giving us some more liberty in the future while making 
the lpae code a bit more generic.
In guest_walk.c there are only one user, so the introduction of an 
helper is quite limited there.
Cheers,

--
Julien Grall

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