Date: Mon, 6 May 2019 13:42:35 -0700 From: Mark Millard <marklmi@yahoo.com> To: Justin Hibbits <chmeeedalf@gmail.com>, FreeBSD PowerPC ML <freebsd-ppc@freebsd.org> Subject: powerpc64: kern_slbtrap double mtlr, one from (PC_SLBSAVE+120)(%r1) and one from (PC_SLBSAVE+136)(%r1)? Message-ID: <E893618A-5734-4D41-9954-825C9D997EB5@yahoo.com>
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kern_slbtrap has . . .
(I have capitalized to make specific MFLR,
MTLR, LD, and STD and related register usage
stand out. Also note PC_SLBSAVE+136 and
PC_SLBSAVE+120 usage.)
CNAME(slbtrap):
. . .
2: MFLR %R2 /* Save the old LR in r2 */
nop
bl 3f /* Begin dance to jump to =
kern_slbtrap*/
.llong kern_slbtrap
3: mflr %r1
ld %r1,0(%r1)
mtlr %r1
GET_CPUINFO(%r1)
blrl /* 124 bytes -- 4 to spare */
CNAME(slbtrapend):
kern_slbtrap:
STD %R2,(PC_SLBSAVE+136)(%R1) /* old LR */
. . .
3: /* Real kernel SLB miss */
. . .
mfxer %r2 /* save XER */
std %r2,(PC_SLBSAVE+112)(%r1)
MFLR %R2 /* save LR (SP already saved) */
STD %R2,(PC_SLBSAVE+120)(%R1)
. . .
LD %R2,(PC_SLBSAVE+120)(%R1)
MTLR %R2
ld %r2,(PC_SLBSAVE+128)(%r1)
mtctr %r2
LD %R2,(PC_SLBSAVE+136)(%R1)
MTLR %R2
/* Restore r0-r3 */
It appears that PC_SLBSAVE+120(%r1) is being used
to save and restore an "internal" lr value,
despite overall being replaced before lr is used.
If I gather right, the ld from PC_SLBSAVE+120(%r1)
is effectively unneeded. May be the store to
PC_SLBSAVE+120(%r1) serves for being able to
see the value in a standard place, but my guess
would be that it is also unneeded in the above
code. Similarly for the matching mtlr and mflr.
=3D=3D=3D
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)
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