[RFC PATCH 1/6] arm64: vmemmap: use virtual projection of linear region

[Will Deacon]

On Fri, Feb 26, 2016 at 04:39:55PM +0100, Ard Biesheuvel wrote:
> On 26 February 2016 at 16:15, Will Deacon <will.deacon at arm.com> wrote:
> > On Thu, Feb 25, 2016 at 08:02:00AM +0100, Ard Biesheuvel wrote:
> >> On 24 February 2016 at 17:21, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> >> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> >> > index a440f5a85d08..8e6baea0ff61 100644
> >> > --- a/arch/arm64/include/asm/pgtable.h
> >> > +++ b/arch/arm64/include/asm/pgtable.h
> >> > @@ -34,18 +34,19 @@
> >> >  /*
> >> >   * VMALLOC and SPARSEMEM_VMEMMAP ranges.
> >> >   *
> >> > - * VMEMAP_SIZE: allows the whole VA space to be covered by a struct page array
> >> > + * VMEMAP_SIZE: allows the whole linear region to be covered by a struct page array
> >> >   *     (rounded up to PUD_SIZE).
> >> >   * VMALLOC_START: beginning of the kernel vmalloc space
> >> >   * VMALLOC_END: extends to the available space below vmmemmap, PCI I/O space,
> >> >   *     fixed mappings and modules
> >> >   */
> >> > -#define VMEMMAP_SIZE           ALIGN((1UL << (VA_BITS - PAGE_SHIFT)) * sizeof(struct page), PUD_SIZE)
> >> > +#define VMEMMAP_SIZE           ALIGN((1UL << (VA_BITS - PAGE_SHIFT - 1)) * sizeof(struct page), PUD_SIZE)
> >> >
> >> >  #define VMALLOC_START          (MODULES_END)
> >> >  #define VMALLOC_END            (PAGE_OFFSET - PUD_SIZE - VMEMMAP_SIZE - SZ_64K)
> >> >
> >> > -#define vmemmap                        ((struct page *)(VMALLOC_END + SZ_64K))
> >> > +#define VMEMMAP_START          (VMALLOC_END + SZ_64K)
> >> > +#define vmemmap                        ((struct page *)(VMEMMAP_START - memstart_addr / sizeof(struct page)))
> >> >
> >>
> >> Note that with the linear region randomization which is now in -next,
> >> this division needs to be signed (since memstart_addr can wrap).
> >>
> >> So I should either update the definition of memstart_addr to s64 in
> >> this patch, or cast to (s64) in the expression above
> >
> > Can you avoid the division altogether by doing something like:
> >
> > (struct page *)(VMEMMAP_START - (PHYS_PFN(memstart_addr) * sizeof(struct page)))
> >
> > or have I misunderstood how this works?
> >
> 
> It needs to be a signed shift, since the RHS of the subtraction must
> remain negative if memstart_addr is 'negative'
> 
> This works as well:
> (struct page *)VMEMMAP_START - ((s64)memstart_addr >> PAGE_SHIFT)

Ah yeah, even better.

> It may be appropriate to change the definition of memstart_addr to
> s64, to reflect that, under randomization of the linear region, the
> start of physical memory may be 'below zero' so that the actual
> populated RAM region is high up in the linear region.
> That way, we can lose the case here.

That sounds like a good idea.

Will