[PATCH 2/2] arm64: use memset to clear BSS

[mark.rutland@arm.com (Mark Rutland)]

On Wed, Jan 06, 2016 at 11:40:39AM +0000, Mark Rutland wrote:
> On Wed, Jan 06, 2016 at 12:12:45PM +0100, Ard Biesheuvel wrote:
> > On 6 January 2016 at 12:05, Mark Rutland <mark.rutland@arm.com> wrote:
> > > Currently we use an open-coded memzero to clear the BSS. As it is a
> > > trivial implementation, it is sub-optimal.
> > >
> > > Our optimised memset doesn't use the stack, is position-independent, and
> > > for the memzero case can use of DC ZVA to clear large blocks
> > > efficiently. In __mmap_switched the MMU is on and there are no live
> > > caller-saved registers, so we can safely call an uninstrumented memset.
> > >
> > > This patch changes __mmap_switched to use memset when clearing the BSS.
> > > We use the __pi_memset alias so as to avoid any instrumentation in all
> > > kernel configurations. As with the head symbols, we must get the linker
> > > to generate __bss_size, as there is no ELF relocation for the
> > > subtraction of two symbols.
> > >
> > > Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> > > Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> > > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > > Cc: Marc Zyngier <marc.zyngier@arm.com>
> > > Cc: Will Deacon <will.deacon@arm.com>
> > > ---
> > >  arch/arm64/kernel/head.S  | 14 ++++++--------
> > >  arch/arm64/kernel/image.h |  2 ++
> > >  2 files changed, 8 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> > > index 23cfc08..247a97b 100644
> > > --- a/arch/arm64/kernel/head.S
> > > +++ b/arch/arm64/kernel/head.S
> > > @@ -415,14 +415,12 @@ ENDPROC(__create_page_tables)
> > >   */
> > >         .set    initial_sp, init_thread_union + THREAD_START_SP
> > >  __mmap_switched:
> > > -       adr_l   x6, __bss_start
> > > -       adr_l   x7, __bss_stop
> > > -
> > > -1:     cmp     x6, x7
> > > -       b.hs    2f
> > > -       str     xzr, [x6], #8                   // Clear BSS
> > > -       b       1b
> > > -2:
> > > +       // clear BSS
> > > +       adr_l   x0, __bss_start
> > > +       mov     x1, xzr
> > > +       mov_l   x2, __bss_size
> > 
> > Is it such a big deal to do
> > 
> > adr_l x2, __bss_stop
> > sub x2, x2, x0
> > 
> > instead?
> 
> I'm happy either way.
> 
> It no-one else has a use for mov_l I'll drop it and move to that.

>From a discussion with Will, it sounds like the sub form is preferable,
so I'll drop mov_l for now.

Thanks,
Mark.