Re: [PATCH v3 05/19] xen/arm: mm: Add support for the contiguous bit

[Julien Grall <julien@xxxxxxx>]

Hi,

On 02/04/2022 00:53, Stefano Stabellini wrote:
> On Mon, 21 Feb 2022, Julien Grall wrote:
> > @@ -1333,21 +1386,34 @@ static int xen_pt_update(unsigned long virt,
> >       while ( left )
> >       {
> >           unsigned int order, level;
> > +        unsigned int nr_contig;
> > +        unsigned int new_flags;
> >   
> >           level = xen_pt_mapping_level(vfn, mfn, left, flags);
> >           order = XEN_PT_LEVEL_ORDER(level);
> >   
> >           ASSERT(left >= BIT(order, UL));
> >   
> > -        rc = xen_pt_update_entry(root, pfn_to_paddr(vfn), mfn, level, flags);
> > -        if ( rc )
> > -            break;
> > +        /*
> > +         * Check if we can set the contiguous mapping and update the
> > +         * flags accordingly.
> > +         */
> > +        nr_contig = xen_pt_check_contig(vfn, mfn, level, left, flags);
> > +        new_flags = flags | ((nr_contig > 1) ? _PAGE_CONTIG : 0);
> Here is an optional idea to make the code simpler. We could move the
> flags changes (adding/removing _PAGE_CONTIG) to xen_pt_check_contig.
> That way, we could remove the inner loop.
>
> xen_pt_check_contig could check if _PAGE_CONTIG is already set and based
> on alignment, it should be able to figure out when it needs to be
> disabled.
My initial attempt was to do everything in a loop. But this didn't pan 
out as I wanted (I felt the code was complex) and there are extra work 
to be done for the next 31 entries (assuming 4KB granularity).
Hence the two loops. Unfortunately, I didn't keep my first attempt. So I 
can't realy show what I wrote.
Cheers,

--
Julien Grall