Re: [PATCH v3 32/60] arm64: head: allocate more pages for the kernel mapping

[Ard Biesheuvel]

On Mon, 17 Apr 2023 at 17:48, Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 07/03/2023 14:04, Ard Biesheuvel wrote:
> > In preparation for switching to an early kernel mapping routine that
> > maps each segment according to its precise boundaries, and with the
> > correct attributes, let's allocate some extra pages for page tables for
> > the 4k page size configuration. This is necessary because the start and
> > end of each segment may not be aligned to the block size, and so we'll
> > need an extra page table at each segment boundary.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > ---
> >  arch/arm64/include/asm/kernel-pgtable.h | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/include/asm/kernel-pgtable.h b/arch/arm64/include/asm/kernel-pgtable.h
> > index 4d13c73171e1e360..50b5c145358a5d8e 100644
> > --- a/arch/arm64/include/asm/kernel-pgtable.h
> > +++ b/arch/arm64/include/asm/kernel-pgtable.h
> > @@ -80,7 +80,7 @@
> >                       + EARLY_PGDS((vstart), (vend), add)     /* each PGDIR needs a next level page table */  \
> >                       + EARLY_PUDS((vstart), (vend), add)     /* each PUD needs a next level page table */    \
> >                       + EARLY_PMDS((vstart), (vend), add))    /* each PMD needs a next level page table */
> > -#define INIT_DIR_SIZE (PAGE_SIZE * EARLY_PAGES(KIMAGE_VADDR, _end, EARLY_KASLR))
> > +#define INIT_DIR_SIZE (PAGE_SIZE * (EARLY_PAGES(KIMAGE_VADDR, _end, EARLY_KASLR) + EARLY_SEGMENT_EXTRA_PAGES))
> >
> >  /* the initial ID map may need two extra pages if it needs to be extended */
> >  #if VA_BITS < 48
> > @@ -101,6 +101,15 @@
> >  #define SWAPPER_TABLE_SHIFT  PMD_SHIFT
> >  #endif
> >
> > +/* The number of segments in the kernel image (text, rodata, inittext, initdata, data+bss) */
> > +#define KERNEL_SEGMENT_COUNT 5
> > +
> > +#if SWAPPER_BLOCK_SIZE > SEGMENT_ALIGN
> > +#define EARLY_SEGMENT_EXTRA_PAGES (KERNEL_SEGMENT_COUNT + 1)
>
> I'm guessing the block size for 4K pages is PMD, so you need these extra pages
> to define PTEs for the case where the section start/end addresses are not on
> exact 2MB boundaries? But in that case, isn't it possible that you would need 2
> extra PTE tables per segment - one for the start and one for the end?
>

The end of one segment is the start of another, so we need one at the
start, plus one each for each segment end.