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

[Ard Biesheuvel]

On Tue, 2 Feb 2021 at 12:36, Catalin Marinas <catalin.marinas at arm.com> wrote:
>
> 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.
>

Never mind, I just got confused. These changes look fine.

For the series,

Acled-by: Ard Biesheuvel <ardb at kernel.org>