Correct the DSCR SPR becoming temporarily corrupted when a task is
context switched when within a transaction. It is corrected when
the transaction is aborted (which will happen after a context switch)
but if the task has suspended (TSUSPEND) the transaction the incorrect
value can be seen.
The problem is caused by saving a thread's DSCR after it has already
been reverted to the CPU's default value:
__switch_to() calls __switch_to_tm()
which calls tm_reclaim_task()
which calls tm_reclaim_thread()
which calls tm_reclaim() where the DSCR is reset
__switch_to() calls _switch
_switch() saves the DSCR to thread.dscr
The fix is to treat the DSCR similarly to the TAR and save it early
in __switch_to().
The program below will expose the problem:
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <asm/tm.h>
#define TBEGIN ".long 0x7C00051D ;"
#define TEND ".long 0x7C00055D ;"
#define TCHECK ".long 0x7C00059C ;"
#define TSUSPEND ".long 0x7C0005DD ;"
#define TRESUME ".long 0x7C2005DD ;"
#define SPRN_TEXASR 0x82
#define SPRN_DSCR 0x03
int main(void) {
uint64_t i = 0, rv, dscr1 = 1, dscr2, texasr;
for (;;) {
rv = 1;
asm __volatile__ (
"ld 3, %[dscr1];"
"mtspr %[sprn_dscr], 3;"
TBEGIN
"beq 1f;"
TSUSPEND
"2: ;"
TCHECK
"bc 4, 0, 2b;"
"mfspr 3, %[sprn_dscr];"
"std 3, %[dscr2];"
"mfspr 3, %[sprn_texasr];"
"std 3, %[texasr];"
TRESUME
TEND
"li %[rv], 0;"
"1: ;"
: [rv]"=r"(rv), [dscr2]"=m"(dscr2), [texasr]"=m"(texasr)
: [dscr1]"m"(dscr1)
, [sprn_dscr]"i"(SPRN_DSCR), [sprn_texasr]"i"(SPRN_TEXASR)
: "memory", "r3"
);
assert(rv);
if ((texasr >> 56) == TM_CAUSE_RESCHED) {
putchar('!');
fflush(stdout);
i++;
}
else {
putchar('.');
fflush(stdout);
}
if (dscr2 != dscr1) {
printf("\n==== DSCR incorrect: 0x%lx (expecting 0x%lx)\n", dscr2,
dscr1);
exit(EXIT_FAILURE);
}
if (i > 10) {
printf("\n==== DSCR TM context switching seems OK.\n");
exit(EXIT_SUCCESS);
}
}
}
Signed-off-by: Sam Bobroff <sam.bobr...@au1.ibm.com>
---
arch/powerpc/include/asm/switch_to.h | 6 ++++--
arch/powerpc/kernel/entry_64.S | 6 ------
arch/powerpc/kernel/process.c | 8 ++++----
3 files changed, 8 insertions(+), 12 deletions(-)
diff --git a/arch/powerpc/include/asm/switch_to.h
b/arch/powerpc/include/asm/switch_to.h
index 2737f46..3efd0e5 100644
--- a/arch/powerpc/include/asm/switch_to.h
+++ b/arch/powerpc/include/asm/switch_to.h
@@ -16,13 +16,15 @@ struct thread_struct;
extern struct task_struct *_switch(struct thread_struct *prev,
struct thread_struct *next);
#ifdef CONFIG_PPC_BOOK3S_64
-static inline void save_tar(struct thread_struct *prev)
+static inline void save_early_sprs(struct thread_struct *prev)
{
if (cpu_has_feature(CPU_FTR_ARCH_207S))
prev->tar = mfspr(SPRN_TAR);
+ if (cpu_has_feature(CPU_FTR_DSCR))
+ prev->dscr = mfspr(SPRN_DSCR);
}
#else
-static inline void save_tar(struct thread_struct *prev) {}
+static inline void save_early_sprs(struct thread_struct *prev) {}
#endif
extern void enable_kernel_fp(void);
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 662c6dd..a107f4a 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -432,12 +432,6 @@ BEGIN_FTR_SECTION
std r24,THREAD_VRSAVE(r3)
END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
#endif /* CONFIG_ALTIVEC */
-#ifdef CONFIG_PPC64
-BEGIN_FTR_SECTION
- mfspr r25,SPRN_DSCR
- std r25,THREAD_DSCR(r3)
-END_FTR_SECTION_IFSET(CPU_FTR_DSCR)
-#endif
and. r0,r0,r22
beq+ 1f
andc r22,r22,r0
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index e247898..8d2065e 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -771,15 +771,15 @@ struct task_struct *__switch_to(struct task_struct *prev,
WARN_ON(!irqs_disabled());
- /* Back up the TAR across context switches.
+ /* Back up the TAR and DSCR across context switches.
* Note that the TAR is not available for use in the kernel. (To
* provide this, the TAR should be backed up/restored on exception
* entry/exit instead, and be in pt_regs. FIXME, this should be in
* pt_regs anyway (for debug).)
- * Save the TAR here before we do treclaim/trecheckpoint as these
- * will change the TAR.
+ * Save the TAR and DSCR here before we do treclaim/trecheckpoint as
+ * these will change them.
*/
- save_tar(&prev->thread);
+ save_early_sprs(&prev->thread);
__switch_to_tm(prev);
--
1.7.10.4
_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@lists.ozlabs.org
https://lists.ozlabs.org/listinfo/linuxppc-dev
