arm64: debug: avoid accessing mdscr_el1 on fault paths where possible

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: Ian Campbell <ian.campbell@citrix.com>
Tested-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

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
 
 /*

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
@@ -275,13 +278,15 @@ 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
 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
@@ -436,25 +438,24 @@ 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
 el0_fpsimd_acc:
 	/*
 	 * Floating Point or Advanced SIMD access
 	 */
+	enable_dbg
 	mov	x0, x25
 	mov	x1, sp
 	b	do_fpsimd_acc
 el0_fpsimd_exc:
 	/*
 	 * Floating Point or Advanced SIMD exception
 	 */
+	enable_dbg
 	mov	x0, x25
 	mov	x1, sp
 	b	do_fpsimd_exc
@@ -463,33 +464,32 @@ 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
 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:
 	/*
 	 * Debug exception handling
 	 */
 	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
@@ -500,30 +500,19 @@ 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
 #endif
 	b	ret_to_user
 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:
@@ -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