Re: [Xen-devel] [PATCH v1] xen/arm: Do not allocate pte entries for MAP_SMALL_PAGES

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Wed, 2015-02-18 at 13:03 +0000, Julien Grall wrote:
> > +                {
> > +                    pte = mfn_to_xen_entry(mfn, (ai & 0xffff));
> 
> Please introduce a new macro for the mask.

Better would be a pte_foo accessor, similar (if not identical) to x86's
pte_get_flags. So pte_get_flags(ai) or so.

> 
> > +                    pte.pt.table = 1;
> > +                    write_pte(&third[third_table_offset(addr)], pte);
> > +                }
> >                   break;
> >               case REMOVE:
> >                   if ( !third[third_table_offset(addr)].pt.valid )
> > diff --git a/xen/include/asm-arm/page.h b/xen/include/asm-arm/page.h
> > index 3e7b0ae..80415b3 100644
> > --- a/xen/include/asm-arm/page.h
> > +++ b/xen/include/asm-arm/page.h
> > @@ -61,10 +61,16 @@
> >   #define DEV_WC        BUFFERABLE
> >   #define DEV_CACHED    WRITEBACK
> >
> > +/* bit 16 in the Attribute index can be used to know if
> > + * PTE entry should be added or not. This is useful
> > + * when ONLY non-leaf page table entries need to allocated
> > + */
> > +#define PTE_INVALID   (0x1 << 16)
> 
> It makes more sense to introduce a PTE_PRESENT flags compare to 
> PTE_INVALID. The former has more meaning that the latter.

Agreed that PTE_PRESENT is the way to go. Ideally this could all be done
via the lpae_pt_t type, but I suspect that might turn out to be tricky.

#define PTE_PRESENT ((struct lpae_t){ .pt.present = 1 }).bits

probably doesn't work, I'm not even sure if this sort of thing is
possible. If not then "#define PTE_PRESET (1ULL<<0)".




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