[PATCH] arm64: debug: avoid accessing mdscr_el1 on fault paths where possible
Will Deacon
will.deacon at arm.com
Fri May 2 08:46:20 PDT 2014
Since mdscr_el1 is part of the debug register group, it is highly likely
to be trapped by a hypervisor to prevent virtual machines from debugging
(buggering?) each other. Unfortunately, this absolutely destroys our
performance, since we access the register on many of our low-level
fault handling paths to keep track of the various debug state machines.
This patch removes our dependency on mdscr_el1 in the case that debugging
is not being used. More specifically we:
- Use TIF_SINGLESTEP to indicate that a task is stepping at EL0 and
avoid disabling step in the MDSCR when we don't need to.
MDSCR_EL1.SS handling is moved to kernel_entry, when trapping from
userspace.
- Ensure debug exceptions are re-enabled on *all* exception entry
paths, even the debug exception handling path (where we re-enable
exceptions after invoking the handler). Since we can now rely on
MDSCR_EL1.SS being cleared by the entry code, exception handlers can
usually enable debug immediately before enabling interrupts.
- Remove all debug exception unmasking from ret_to_user and
el1_preempt, since we will never get here with debug exceptions
masked.
This results in a slight change to kernel debug behaviour, where we now
step into interrupt handlers and data aborts from EL1 when debugging the
kernel, which is actually a useful thing to do. A side-effect of this is
that it *does* potentially prevent stepping off {break,watch}points when
there is a high-frequency interrupt source (e.g. a timer), so a debugger
would need to use either breakpoints or manually disable interrupts to
get around this issue.
With this patch applied, guest performance is restored under KVM when
debug register accesses are trapped (and we get a measurable performance
increase on the host on Cortex-A57 too).
Cc: Marc Zyngier <marc.zyngier at arm.com>
Signed-off-by: Will Deacon <will.deacon at arm.com>
---
arch/arm64/include/asm/assembler.h | 23 ++++++++----
arch/arm64/kernel/entry.S | 73 ++++++++++++++------------------------
2 files changed, 42 insertions(+), 54 deletions(-)
diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index fd3e3924041b..5901480bfdca 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -21,6 +21,7 @@
#endif
#include <asm/ptrace.h>
+#include <asm/thread_info.h>
/*
* Stack pushing/popping (register pairs only). Equivalent to store decrement
@@ -68,23 +69,31 @@
msr daifclr, #8
.endm
- .macro disable_step, tmp
+ .macro disable_step_tsk, flgs, tmp
+ tbz \flgs, #TIF_SINGLESTEP, 9990f
mrs \tmp, mdscr_el1
bic \tmp, \tmp, #1
msr mdscr_el1, \tmp
+ isb // Synchronise with enable_dbg
+9990:
.endm
- .macro enable_step, tmp
+ .macro enable_step_tsk, flgs, tmp
+ tbz \flgs, #TIF_SINGLESTEP, 9990f
+ disable_dbg
mrs \tmp, mdscr_el1
orr \tmp, \tmp, #1
msr mdscr_el1, \tmp
+9990:
.endm
- .macro enable_dbg_if_not_stepping, tmp
- mrs \tmp, mdscr_el1
- tbnz \tmp, #0, 9990f
- enable_dbg
-9990:
+/*
+ * Enable both debug exceptions and interrupts. This is likely to be
+ * faster than two daifclr operations, since writes to this register
+ * are self-synchronising.
+ */
+ .macro enable_dbg_and_irq
+ msr daifclr, #(8 | 2)
.endm
/*
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 39ac630d83de..e8b23a3b68d0 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -60,6 +60,9 @@
push x0, x1
.if \el == 0
mrs x21, sp_el0
+ get_thread_info tsk // Ensure MDSCR_EL1.SS is clear,
+ ldr x19, [tsk, #TI_FLAGS] // since we can unmask debug
+ disable_step_tsk x19, x20 // exceptions when scheduling.
.else
add x21, sp, #S_FRAME_SIZE
.endif
@@ -259,7 +262,7 @@ el1_da:
* Data abort handling
*/
mrs x0, far_el1
- enable_dbg_if_not_stepping x2
+ enable_dbg
// re-enable interrupts if they were enabled in the aborted context
tbnz x23, #7, 1f // PSR_I_BIT
enable_irq
@@ -275,6 +278,7 @@ el1_sp_pc:
* Stack or PC alignment exception handling
*/
mrs x0, far_el1
+ enable_dbg
mov x1, x25
mov x2, sp
b do_sp_pc_abort
@@ -282,6 +286,7 @@ el1_undef:
/*
* Undefined instruction
*/
+ enable_dbg
mov x0, sp
b do_undefinstr
el1_dbg:
@@ -294,10 +299,11 @@ el1_dbg:
mrs x0, far_el1
mov x2, sp // struct pt_regs
bl do_debug_exception
-
+ enable_dbg
kernel_exit 1
el1_inv:
// TODO: add support for undefined instructions in kernel mode
+ enable_dbg
mov x0, sp
mov x1, #BAD_SYNC
mrs x2, esr_el1
@@ -307,7 +313,7 @@ ENDPROC(el1_sync)
.align 6
el1_irq:
kernel_entry 1
- enable_dbg_if_not_stepping x0
+ enable_dbg
#ifdef CONFIG_TRACE_IRQFLAGS
bl trace_hardirqs_off
#endif
@@ -332,8 +338,7 @@ ENDPROC(el1_irq)
#ifdef CONFIG_PREEMPT
el1_preempt:
mov x24, lr
-1: enable_dbg
- bl preempt_schedule_irq // irq en/disable is done inside
+1: bl preempt_schedule_irq // irq en/disable is done inside
ldr x0, [tsk, #TI_FLAGS] // get new tasks TI_FLAGS
tbnz x0, #TIF_NEED_RESCHED, 1b // needs rescheduling?
ret x24
@@ -349,7 +354,7 @@ el0_sync:
lsr x24, x25, #ESR_EL1_EC_SHIFT // exception class
cmp x24, #ESR_EL1_EC_SVC64 // SVC in 64-bit state
b.eq el0_svc
- adr lr, ret_from_exception
+ adr lr, ret_to_user
cmp x24, #ESR_EL1_EC_DABT_EL0 // data abort in EL0
b.eq el0_da
cmp x24, #ESR_EL1_EC_IABT_EL0 // instruction abort in EL0
@@ -378,7 +383,7 @@ el0_sync_compat:
lsr x24, x25, #ESR_EL1_EC_SHIFT // exception class
cmp x24, #ESR_EL1_EC_SVC32 // SVC in 32-bit state
b.eq el0_svc_compat
- adr lr, ret_from_exception
+ adr lr, ret_to_user
cmp x24, #ESR_EL1_EC_DABT_EL0 // data abort in EL0
b.eq el0_da
cmp x24, #ESR_EL1_EC_IABT_EL0 // instruction abort in EL0
@@ -423,11 +428,8 @@ el0_da:
*/
mrs x0, far_el1
bic x0, x0, #(0xff << 56)
- disable_step x1
- isb
- enable_dbg
// enable interrupts before calling the main handler
- enable_irq
+ enable_dbg_and_irq
mov x1, x25
mov x2, sp
b do_mem_abort
@@ -436,11 +438,8 @@ el0_ia:
* Instruction abort handling
*/
mrs x0, far_el1
- disable_step x1
- isb
- enable_dbg
// enable interrupts before calling the main handler
- enable_irq
+ enable_dbg_and_irq
orr x1, x25, #1 << 24 // use reserved ISS bit for instruction aborts
mov x2, sp
b do_mem_abort
@@ -448,6 +447,7 @@ el0_fpsimd_acc:
/*
* Floating Point or Advanced SIMD access
*/
+ enable_dbg
mov x0, x25
mov x1, sp
b do_fpsimd_acc
@@ -455,6 +455,7 @@ el0_fpsimd_exc:
/*
* Floating Point or Advanced SIMD exception
*/
+ enable_dbg
mov x0, x25
mov x1, sp
b do_fpsimd_exc
@@ -463,11 +464,8 @@ el0_sp_pc:
* Stack or PC alignment exception handling
*/
mrs x0, far_el1
- disable_step x1
- isb
- enable_dbg
// enable interrupts before calling the main handler
- enable_irq
+ enable_dbg_and_irq
mov x1, x25
mov x2, sp
b do_sp_pc_abort
@@ -475,9 +473,9 @@ el0_undef:
/*
* Undefined instruction
*/
- mov x0, sp
// enable interrupts before calling the main handler
- enable_irq
+ enable_dbg_and_irq
+ mov x0, sp
b do_undefinstr
el0_dbg:
/*
@@ -485,11 +483,13 @@ el0_dbg:
*/
tbnz x24, #0, el0_inv // EL0 only
mrs x0, far_el1
- disable_step x1
mov x1, x25
mov x2, sp
- b do_debug_exception
+ bl do_debug_exception
+ enable_dbg
+ b ret_to_user
el0_inv:
+ enable_dbg
mov x0, sp
mov x1, #BAD_SYNC
mrs x2, esr_el1
@@ -500,15 +500,12 @@ ENDPROC(el0_sync)
el0_irq:
kernel_entry 0
el0_irq_naked:
- disable_step x1
- isb
enable_dbg
#ifdef CONFIG_TRACE_IRQFLAGS
bl trace_hardirqs_off
#endif
irq_handler
- get_thread_info tsk
#ifdef CONFIG_TRACE_IRQFLAGS
bl trace_hardirqs_on
@@ -517,14 +514,6 @@ el0_irq_naked:
ENDPROC(el0_irq)
/*
- * This is the return code to user mode for abort handlers
- */
-ret_from_exception:
- get_thread_info tsk
- b ret_to_user
-ENDPROC(ret_from_exception)
-
-/*
* Register switch for AArch64. The callee-saved registers need to be saved
* and restored. On entry:
* x0 = previous task_struct (must be preserved across the switch)
@@ -563,10 +552,7 @@ ret_fast_syscall:
ldr x1, [tsk, #TI_FLAGS]
and x2, x1, #_TIF_WORK_MASK
cbnz x2, fast_work_pending
- tbz x1, #TIF_SINGLESTEP, fast_exit
- disable_dbg
- enable_step x2
-fast_exit:
+ enable_step_tsk x1, x2
kernel_exit 0, ret = 1
/*
@@ -585,7 +571,6 @@ work_pending:
bl do_notify_resume
b ret_to_user
work_resched:
- enable_dbg
bl schedule
/*
@@ -596,9 +581,7 @@ ret_to_user:
ldr x1, [tsk, #TI_FLAGS]
and x2, x1, #_TIF_WORK_MASK
cbnz x2, work_pending
- tbz x1, #TIF_SINGLESTEP, no_work_pending
- disable_dbg
- enable_step x2
+ enable_step_tsk x1, x2
no_work_pending:
kernel_exit 0, ret = 0
ENDPROC(ret_to_user)
@@ -625,12 +608,8 @@ el0_svc:
mov sc_nr, #__NR_syscalls
el0_svc_naked: // compat entry point
stp x0, scno, [sp, #S_ORIG_X0] // save the original x0 and syscall number
- disable_step x16
- isb
- enable_dbg
- enable_irq
+ enable_dbg_and_irq
- get_thread_info tsk
ldr x16, [tsk, #TI_FLAGS] // check for syscall tracing
tbnz x16, #TIF_SYSCALL_TRACE, __sys_trace // are we tracing syscalls?
adr lr, ret_fast_syscall // return address
--
1.9.2
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