[PATCH 3/6] s390: add options to change branch prediction behaviour for the kernel

Martin Schwidefsky
2018-01-17T09:48:36Z

Add the PPA instruction to the system entry and exit path to switch
the kernel to a different branch prediction behaviour. The instructions
are added via CPU alternatives and can be disabled with the "nospec"
or the "nobp=0" kernel parameter. If the default behaviour selected
with CONFIG_KERNEL_NOBP is set to "n" then the "nobp=1" parameter can be
used to enable the changed kernel branch prediction.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
---
arch/s390/Kconfig | 17 +++++++++++++
arch/s390/include/asm/processor.h | 1 +
arch/s390/kernel/alternative.c | 23 ++++++++++++++++++
arch/s390/kernel/early.c | 2 ++
arch/s390/kernel/entry.S | 50 ++++++++++++++++++++++++++++++++++++++-
arch/s390/kernel/ipl.c | 1 +
arch/s390/kernel/smp.c | 2 ++
7 files changed, 95 insertions(+), 1 deletion(-)

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 829c679..a818644 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -541,6 +541,23 @@ config ARCH_RANDOM

If unsure, say Y.

+config KERNEL_NOBP
+ def_bool n
+ prompt "Enable modified branch prediction for the kernel by default"
+ help
+ If this option is selected the kernel will switch to a modified
+ branch prediction mode if the firmware interface is available.
+ The modified branch prediction mode improves the behaviour in
+ regard to speculative execution.
+
+ With the option enabled the kernel parameter "nobp=0" or "nospec"
+ can be used to run the kernel in the normal branch prediction mode.
+
+ With the option disabled the modified branch prediction mode is
+ enabled with the "nobp=1" kernel parameter.
+
+ If unsure, say N.
+
endmenu

menu "Memory setup"
diff --git a/arch/s390/include/asm/processor.h b/arch/s390/include/asm/processor.h
index bfbfad4..5f37f9c 100644
--- a/arch/s390/include/asm/processor.h
+++ b/arch/s390/include/asm/processor.h
@@ -91,6 +91,7 @@ void cpu_detect_mhz_feature(void);
extern const struct seq_operations cpuinfo_op;
extern int sysctl_ieee_emulation_warnings;
extern void execve_tail(void);
+extern void __bpon(void);

/*
* User space process size: 2GB for 31 bit, 4TB or 8PT for 64 bit.
diff --git a/arch/s390/kernel/alternative.c b/arch/s390/kernel/alternative.c
index 33d2e88..2546bfb 100644
--- a/arch/s390/kernel/alternative.c
+++ b/arch/s390/kernel/alternative.c
@@ -15,6 +15,29 @@ static int __init disable_alternative_instructions(char *str)

early_param("noaltinstr", disable_alternative_instructions);

+static int __init nobp_setup_early(char *str)
+{
+ bool enabled;
+ int rc;
+
+ rc = kstrtobool(str, &enabled);
+ if (rc)
+ return rc;
+ if (enabled && test_facility(82))
+ __set_facility(82, S390_lowcore.alt_stfle_fac_list);
+ else
+ __clear_facility(82, S390_lowcore.alt_stfle_fac_list);
+ return 0;
+}
+early_param("nobp", nobp_setup_early);
+
+static int __init nospec_setup_early(char *str)
+{
+ __clear_facility(82, S390_lowcore.alt_stfle_fac_list);
+ return 0;
+}
+early_param("nospec", nospec_setup_early);
+
static int __init nogmb_setup_early(char *str)
{
__clear_facility(81, S390_lowcore.alt_stfle_fac_list);
diff --git a/arch/s390/kernel/early.c b/arch/s390/kernel/early.c
index 510f218..ac707a9 100644
--- a/arch/s390/kernel/early.c
+++ b/arch/s390/kernel/early.c
@@ -196,6 +196,8 @@ static noinline __init void setup_facility_list(void)
memcpy(S390_lowcore.alt_stfle_fac_list,
S390_lowcore.stfle_fac_list,
sizeof(S390_lowcore.alt_stfle_fac_list));
+ if (!IS_ENABLED(CONFIG_KERNEL_NOBP))
+ __clear_facility(82, S390_lowcore.alt_stfle_fac_list);
}

static __init void detect_diag9c(void)
diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
index 9e5f6cd..dab716b 100644
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -159,6 +159,34 @@ _PIF_WORK = (_PIF_PER_TRAP | _PIF_SYSCALL_RESTART)
tm off+\addr, \mask
.endm

+ .macro BPOFF
+ .pushsection .altinstr_replacement, "ax"
+660: .long 0xb2e8c000
+ .popsection
+661: .long 0x47000000
+ .pushsection .altinstructions, "a"
+ .long 661b - .
+ .long 660b - .
+ .word 82
+ .byte 4
+ .byte 4
+ .popsection
+ .endm
+
+ .macro BPON
+ .pushsection .altinstr_replacement, "ax"
+662: .long 0xb2e8d000
+ .popsection
+663: .long 0x47000000
+ .pushsection .altinstructions, "a"
+ .long 663b - .
+ .long 662b - .
+ .word 82
+ .byte 4
+ .byte 4
+ .popsection
+ .endm
+
.section .kprobes.text, "ax"
.Ldummy:
/*
@@ -171,6 +199,11 @@ _PIF_WORK = (_PIF_PER_TRAP | _PIF_SYSCALL_RESTART)
*/
nop 0

+ENTRY(__bpon)
+ .globl __bpon
+ BPON
+ br %r14
+
/*
* Scheduler resume function, called by switch_to
* gpr2 = (task_struct *) prev
@@ -226,8 +259,11 @@ ENTRY(sie64a)
jnz .Lsie_skip
TSTMSK __LC_CPU_FLAGS,_CIF_FPU
jo .Lsie_skip # exit if fp/vx regs changed
+ BPON
.Lsie_entry:
sie 0(%r14)
+.Lsie_exit:
+ BPOFF
.Lsie_skip:
ni __SIE_PROG0C+3(%r14),0xfe # no longer in SIE
lctlg %c1,%c1,__LC_USER_ASCE # load primary asce
@@ -273,6 +309,7 @@ ENTRY(system_call)
stpt __LC_SYNC_ENTER_TIMER
.Lsysc_stmg:
stmg %r8,%r15,__LC_SAVE_AREA_SYNC
+ BPOFF
lg %r12,__LC_CURRENT
lghi %r13,__TASK_thread
lghi %r14,_PIF_SYSCALL
@@ -317,6 +354,7 @@ ENTRY(system_call)
jnz .Lsysc_work # check for work
TSTMSK __LC_CPU_FLAGS,_CIF_WORK
jnz .Lsysc_work
+ BPON
.Lsysc_restore:
lg %r14,__LC_VDSO_PER_CPU
lmg %r0,%r10,__PT_R0(%r11)
@@ -522,6 +560,7 @@ ENTRY(kernel_thread_starter)

ENTRY(pgm_check_handler)
stpt __LC_SYNC_ENTER_TIMER
+ BPOFF
stmg %r8,%r15,__LC_SAVE_AREA_SYNC
lg %r10,__LC_LAST_BREAK
lg %r12,__LC_CURRENT
@@ -620,6 +659,7 @@ ENTRY(pgm_check_handler)
ENTRY(io_int_handler)
STCK __LC_INT_CLOCK
stpt __LC_ASYNC_ENTER_TIMER
+ BPOFF
stmg %r8,%r15,__LC_SAVE_AREA_ASYNC
lg %r12,__LC_CURRENT
larl %r13,cleanup_critical
@@ -660,9 +700,13 @@ ENTRY(io_int_handler)
lg %r14,__LC_VDSO_PER_CPU
lmg %r0,%r10,__PT_R0(%r11)
mvc __LC_RETURN_PSW(16),__PT_PSW(%r11)
+ tm __PT_PSW+1(%r11),0x01 # returning to user ?
+ jno .Lio_exit_kernel
+ BPON
.Lio_exit_timer:
stpt __LC_EXIT_TIMER
mvc __VDSO_ECTG_BASE(16,%r14),__LC_EXIT_TIMER
+.Lio_exit_kernel:
lmg %r11,%r15,__PT_R11(%r11)
lpswe __LC_RETURN_PSW
.Lio_done:
@@ -833,6 +877,7 @@ ENTRY(io_int_handler)
ENTRY(ext_int_handler)
STCK __LC_INT_CLOCK
stpt __LC_ASYNC_ENTER_TIMER
+ BPOFF
stmg %r8,%r15,__LC_SAVE_AREA_ASYNC
lg %r12,__LC_CURRENT
larl %r13,cleanup_critical
@@ -871,6 +916,7 @@ ENTRY(psw_idle)
.Lpsw_idle_stcctm:
#endif
oi __LC_CPU_FLAGS+7,_CIF_ENABLED_WAIT
+ BPON
STCK __CLOCK_IDLE_ENTER(%r2)
stpt __TIMER_IDLE_ENTER(%r2)
.Lpsw_idle_lpsw:
@@ -971,6 +1017,7 @@ load_fpu_regs:
*/
ENTRY(mcck_int_handler)
STCK __LC_MCCK_CLOCK
+ BPOFF
la %r1,4095 # validate r1
spt __LC_CPU_TIMER_SAVE_AREA-4095(%r1) # validate cpu timer
sckc __LC_CLOCK_COMPARATOR # validate comparator
@@ -1071,6 +1118,7 @@ ENTRY(mcck_int_handler)
mvc __LC_RETURN_MCCK_PSW(16),__PT_PSW(%r11) # move return PSW
tm __LC_RETURN_MCCK_PSW+1,0x01 # returning to user ?
jno 0f
+ BPON
stpt __LC_EXIT_TIMER
mvc __VDSO_ECTG_BASE(16,%r14),__LC_EXIT_TIMER
0: lmg %r11,%r15,__PT_R11(%r11)
@@ -1385,7 +1433,7 @@ cleanup_critical:
.Lsie_crit_mcck_start:
.quad .Lsie_entry
.Lsie_crit_mcck_length:
- .quad .Lsie_skip - .Lsie_entry
+ .quad .Lsie_exit - .Lsie_entry
#endif

.section .rodata, "a"
diff --git a/arch/s390/kernel/ipl.c b/arch/s390/kernel/ipl.c
index 8ecb872..443b9b8 100644
--- a/arch/s390/kernel/ipl.c
+++ b/arch/s390/kernel/ipl.c
@@ -547,6 +547,7 @@ static struct kset *ipl_kset;

static void __ipl_run(void *unused)
{
+ __bpon();
diag308(DIAG308_LOAD_CLEAR, NULL);
if (MACHINE_IS_VM)
__cpcmd("IPL", NULL, 0, NULL);
diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
index 4ce68ca..9cd1696 100644
--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c
@@ -319,6 +319,7 @@ static void pcpu_delegate(struct pcpu *pcpu, void (*func)(void *),
mem_assign_absolute(lc->restart_fn, (unsigned long) func);
mem_assign_absolute(lc->restart_data, (unsigned long) data);
mem_assign_absolute(lc->restart_source, source_cpu);
+ __bpon();
asm volatile(
"0: sigp 0,%0,%2 # sigp restart to target cpu\n"
" brc 2,0b # busy, try again\n"
@@ -903,6 +904,7 @@ void __cpu_die(unsigned int cpu)
void __noreturn cpu_die(void)
{
idle_task_exit();
+ __bpon();
pcpu_sigp_retry(pcpu_devices + smp_processor_id(), SIGP_STOP, 0);
for (;;) ;
}
--
2.7.4