Date: Fri, 10 Oct 2014 08:04:35 -0700 From: Mark Millard <markmi@dsl-only.net> To: FreeBSD PowerPC ML <freebsd-ppc@freebsd.org> Cc: Justin Hibbits <chmeeedalf@gmail.com> Subject: powerpc64/GENERIC64: s_trap vs. realtrap, why does realtrap possibly execute restore_kernsrs twice in a row overall? Message-ID: <511C0B85-173F-4863-ACA5-0274C381E005@dsl-only.net>
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Based on 10.1-RC1 sources but probably long true..
s_trap in /usr/src/sys/powerpc/aim/trap_subr64.S starts with the =
following sequence of 8 lines of source immediately after the s_trap =
label (ignoring the unused u_trap label line, not shown below):
s_trap:
bf 17,k_trap /* branch if PSL_PR is false */
GET_CPUINFO(%r1)
ld %r1,PC_CURPCB(%r1)
mr %r27,%r28 /* Save LR, r29 */
mtsprg2 %r29
bl restore_kernsrs /* enable kernel mapping */
mfsprg2 %r29
mr %r28,%r27
realtrap also has that exact instruction sequence starting a few =
instructions down in its code --but immeadiately followed by a branch to =
s_strap that ends up reexecuting that same instruction sequence (but =
in/at s_trap), presuming that the original "bd 17,k_trap" in realtrap is =
not taken:
realtrap:
/* Test whether we already had PR set */
mfsrr1 %r1
mtcr %r1
mfsprg1 %r1 /* restore SP (might have been
overwritten) */
bf 17,k_trap /* branch if PSL_PR is false */
GET_CPUINFO(%r1)
ld %r1,PC_CURPCB(%r1)
mr %r27,%r28 /* Save LR, r29 */
mtsprg2 %r29
bl restore_kernsrs /* enable kernel mapping */
mfsprg2 %r29
mr %r28,%r27
ba s_trap
Why would this code for possibly calling restore_kernsrs possibly be =
executed twice back-to-back for realtrap's execution overall? Is this =
deliberate for some reason? Could the realtrap code validly avoid the =
doubled activity when it does not immediately jump to k_trap?
Note that if it was possible for s_trap to take the "bf 17,k_trap" =
branch after realtrap did its "ba s_trap" then %r1 for that s_trap use =
would not have the stack pointer in it for k_trap's potential use (e.g., =
in its use of FRAME_SETUP). restore_kernsrs does not touch %r1 and =
k_trap takes %r1 as-is for use as a stack pointer in FRAME_SETUP. =
Luckily it appears that s_trap would not take that branch in this =
context.
But if that last is true that means that s_trap's code definitely always =
does call restore_kernsrs a second time when used from realtrap. =
Wasteful?
[And for realtrap it would seem that the "mfsrr1 %r1" definitely "might =
have" overwritten %r1 (a.k.a. the Stack Pointer): The "might have" =
comment seems a little odd. It is more like the SP would be needed in =
the branch to k_trap case and so had to be put back in %r1 to cover =
that.]
=3D=3D=3D
Mark Millard
markmi at dsl-only.net
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