[PATCH 2/2] arm64: Use simpler arithmetics for the linear map macros

[Catalin Marinas]

On Tue, Feb 02, 2021 at 12:05:10AM +0100, Ard Biesheuvel wrote:
> On Mon, 1 Feb 2021 at 20:06, Catalin Marinas <catalin.marinas at arm.com> wrote:
> > Because of the tagged addresses, the __is_lm_address() and
> > __lm_to_phys() macros grew to some harder to understand bitwise
> > operations using PAGE_OFFSET. Since these macros only accept untagged
> > addresses, use a simple subtract operation.
> >
> > Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
> > Cc: Will Deacon <will at kernel.org>
> > Cc: Ard Biesheuvel <ardb at kernel.org>
> > Cc: Mark Rutland <mark.rutland at arm.com>
> > ---
> >  arch/arm64/include/asm/memory.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> > index 3c1aaa522cbd..ff4732785c32 100644
> > --- a/arch/arm64/include/asm/memory.h
> > +++ b/arch/arm64/include/asm/memory.h
> > @@ -251,9 +251,9 @@ static inline const void *__tag_set(const void *addr, u8 tag)
> >   * lives in the [PAGE_OFFSET, PAGE_END) interval at the bottom of the
> >   * kernel's TTBR1 address range.
> >   */
> > -#define __is_lm_address(addr)  (((u64)(addr) ^ PAGE_OFFSET) < (PAGE_END - PAGE_OFFSET))
> > +#define __is_lm_address(addr)  (((u64)(addr) - PAGE_OFFSET) < (PAGE_END - PAGE_OFFSET))
> 
> Why still subtract PAGE_OFFSET on both sides of the comparison here?

Not sure what you mean (Linus replied on the local variable aspect).

With signed arithmetics, the above would be equivalent to addr < PAGE_END
but it wouldn't cover the addr < PAGE_OFFSET case. So we rely on the
unsigned wrap-around to detect this.

-- 
Catalin