[PATCH 2/2] arm64: Extend early page table code to allow for larger kernels

[Steve Capper]

On Mon, Nov 20, 2017 at 05:00:10PM +0000, Mark Rutland wrote:
> Hi,

Hi Mark,

> 
> On Fri, Nov 17, 2017 at 11:41:43AM +0000, Steve Capper wrote:
> > -#define SWAPPER_DIR_SIZE	(SWAPPER_PGTABLE_LEVELS * PAGE_SIZE)
> > +#define EARLY_PGDS(vstart, vend) ((vend >> PGDIR_SHIFT) - (vstart >> PGDIR_SHIFT) + 1)
> > +
> > +#if SWAPPER_PGTABLE_LEVELS > 3
> > +#define EARLY_PUDS(vstart, vend) ((vend >> PUD_SHIFT) - (vstart >> PUD_SHIFT) + 1)
> > +#else
> > +#define EARLY_PUDS(vstart, vend) (0)
> > +#endif
> > +
> > +#if SWAPPER_PGTABLE_LEVELS > 2
> > +#define EARLY_PMDS(vstart, vend) ((vend >> PMD_SHIFT) - (vstart >> PMD_SHIFT) + 1)
> > +#else
> > +#define EARLY_PMDS(vstart, vend) (0)
> > +#endif
> > +
> > +#define EARLY_PAGES(vstart, vend) ( 1 			/* PGDIR page */				\
> > +			+ EARLY_PGDS((vstart), (vend)) 	/* each PGDIR needs a next level page table */	\
> > +			+ EARLY_PUDS((vstart), (vend))	/* each PUD needs a next level page table */	\
> > +			+ EARLY_PMDS((vstart), (vend)))	/* each PMD needs a next level page table */
> > +#define SWAPPER_DIR_SIZE (PAGE_SIZE * EARLY_PAGES(KIMAGE_VADDR + TEXT_OFFSET, _end + SZ_2M))
> >  #define IDMAP_DIR_SIZE		(IDMAP_PGTABLE_LEVELS * PAGE_SIZE)
> 
> I'm currently struggling to convince myself as to whether 2M is
> necessary/sufficient slack space for all configurations.

Agreed, if the possible address range is changed outside these bounds
then we need to extend them.

> 
> At least w.r.t. Ard's comment, we'd need a bit more slack to allow KASLR
> to cross PGD/PUD/PMD boundaries.
> 
> For example with 3 levels of 64K pages, and a huge kernel that takes up
> SZ_512M - SZ_2M - TEXT_OFFSET. That perfectly fits into 1 PGDIR, 1 PGD,
> and 1 PMD. IIUC, the above would allocate us 3 pages for this case. If
> the kernel were to be relocated such that it straddled two PGDs, we'd
> need 2 PGDs and 2 PMDs, needing 5 pages total.
> 
> I'm not sure that we have a problem if we don't relax KASLR.
> 

The approach I've adopted is to compute which indices are required for
PGDs, PUDs, PMDs to map the supplied address range, then count them.
If we require two PGD entries then that means we need two pages
containing PMD entries to be allocated.

If we consider just PGDs, for example, the only way I am aware of the
kernel straddling more PGDs than previously computed is for the mapping
to begin before vstart or end after vend (or both).

Should I refine the range specified in the SWAPPER_DIR_SIZE for the
current KASLR? (I thought the random offset was < SZ_2M?)

Cheers,
-- 
Steve