Re: [PATCH V2 2/4] powerpc: add helper to check if offset is within rel branch range

[Anju T Sudhakar]

Hi Masami,


Thank you for reviewing the patch set.


On Friday 16 December 2016 05:22 PM, Masami Hiramatsu wrote:
> On Wed, 14 Dec 2016 21:48:30 +0530
> Anju T Sudhakar <anju@xxxxxxxxxxxxxxxxxx> wrote:
>
> > From: "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
> The coding is OK to me. Please add a description for this patch
> here, e.g. what is done by this patch, what kind of branch
> instruction will be covered, and why thse checks are needed etc.


Sure. I will give a description for this patch.


Thanks and Regards,

-Anju

> Thank you,
>
> > Signed-off-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Anju T Sudhakar <anju@xxxxxxxxxxxxxxxxxx>
> > ---
> >   arch/powerpc/include/asm/code-patching.h |  1 +
> >   arch/powerpc/lib/code-patching.c         | 24 +++++++++++++++++++++++-
> >   2 files changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/powerpc/include/asm/code-patching.h b/arch/powerpc/include/asm/code-patching.h
> > index 2015b07..75ee4f4 100644
> > --- a/arch/powerpc/include/asm/code-patching.h
> > +++ b/arch/powerpc/include/asm/code-patching.h
> > @@ -22,6 +22,7 @@
> >   #define BRANCH_SET_LINK	0x1
> >   #define BRANCH_ABSOLUTE	0x2
> >   
> > +bool is_offset_in_branch_range(long offset);
> >   unsigned int create_branch(const unsigned int *addr,
> >   			   unsigned long target, int flags);
> >   unsigned int create_cond_branch(const unsigned int *addr,
> > diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
> > index d5edbeb..f643451 100644
> > --- a/arch/powerpc/lib/code-patching.c
> > +++ b/arch/powerpc/lib/code-patching.c
> > @@ -32,6 +32,28 @@ int patch_branch(unsigned int *addr, unsigned long target, int flags)
> >   	return patch_instruction(addr, create_branch(addr, target, flags));
> >   }
> >   
> > +bool is_offset_in_branch_range(long offset)
> > +{
> > +	/*
> > +	 * Powerpc branch instruction is :
> > +	 *
> > +	 *  0         6                 30   31
> > +	 *  +---------+----------------+---+---+
> > +	 *  | opcode  |     LI         |AA |LK |
> > +	 *  +---------+----------------+---+---+
> > +	 *  Where AA = 0 and LK = 0
> > +	 *
> > +	 * LI is a signed 24 bits integer. The real branch offset is computed
> > +	 * by: imm32 = SignExtend(LI:'0b00', 32);
> > +	 *
> > +	 * So the maximum forward branch should be:
> > +	 *   (0x007fffff << 2) = 0x01fffffc =  0x1fffffc
> > +	 * The maximum backward branch should be:
> > +	 *   (0xff800000 << 2) = 0xfe000000 = -0x2000000
> > +	 */
> > +	return (offset >= -0x2000000 && offset <= 0x1fffffc && !(offset & 0x3));
> > +}
> > +
> >   unsigned int create_branch(const unsigned int *addr,
> >   			   unsigned long target, int flags)
> >   {
> > @@ -43,7 +65,7 @@ unsigned int create_branch(const unsigned int *addr,
> >   		offset = offset - (unsigned long)addr;
> >   
> >   	/* Check we can represent the target in the instruction format */
> > -	if (offset < -0x2000000 || offset > 0x1fffffc || offset & 0x3)
> > +	if (!is_offset_in_branch_range(offset))
> >   		return 0;
> >   
> >   	/* Mask out the flags and target, so they don't step on each other. */
> > --
> > 2.7.4