arm64: entry: consolidate Cortex-A76 erratum 1463225 workaround

The workaround for Cortex-A76 erratum 1463225 is split across the
syscall and debug handlers in separate files. This structure currently
forces us to do some redundant work for debug exceptions from EL0, is a
little difficult to follow, and gets in the way of some future rework of
the exception entry code as it requires exceptions to be unmasked late
in the syscall handling path.

To simplify things, and as a preparatory step for future rework of
exception entry, this patch moves all the workaround logic into
entry-common.c. As the debug handler only needs to run for EL1 debug
exceptions, we no longer call it for EL0 debug exceptions, and no longer
need to check user_mode(regs) as this is always false. For clarity
cortex_a76_erratum_1463225_debug_handler() is changed to return bool.

In the SVC path, the workaround is applied earlier, but this should have
no functional impact as exceptions are still masked. In the debug path
we run the fixup before explicitly disabling preemption, but we will not
attempt to preempt before returning from the exception.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210202120341.28858-1-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 53 insertions, 1 deletions

diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index 5346953e4382..9d3588450473 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -109,6 +109,55 @@ asmlinkage void noinstr exit_el1_irq_or_nmi(struct pt_regs *regs)
 		exit_to_kernel_mode(regs);
 }
 
+#ifdef CONFIG_ARM64_ERRATUM_1463225
+static DEFINE_PER_CPU(int, __in_cortex_a76_erratum_1463225_wa);
+
+static void cortex_a76_erratum_1463225_svc_handler(void)
+{
+	u32 reg, val;
+
+	if (!unlikely(test_thread_flag(TIF_SINGLESTEP)))
+		return;
+
+	if (!unlikely(this_cpu_has_cap(ARM64_WORKAROUND_1463225)))
+		return;
+
+	__this_cpu_write(__in_cortex_a76_erratum_1463225_wa, 1);
+	reg = read_sysreg(mdscr_el1);
+	val = reg | DBG_MDSCR_SS | DBG_MDSCR_KDE;
+	write_sysreg(val, mdscr_el1);
+	asm volatile("msr daifclr, #8");
+	isb();
+
+	/* We will have taken a single-step exception by this point */
+
+	write_sysreg(reg, mdscr_el1);
+	__this_cpu_write(__in_cortex_a76_erratum_1463225_wa, 0);
+}
+
+static bool cortex_a76_erratum_1463225_debug_handler(struct pt_regs *regs)
+{
+	if (!__this_cpu_read(__in_cortex_a76_erratum_1463225_wa))
+		return false;
+
+	/*
+	 * We've taken a dummy step exception from the kernel to ensure
+	 * that interrupts are re-enabled on the syscall path. Return back
+	 * to cortex_a76_erratum_1463225_svc_handler() with debug exceptions
+	 * masked so that we can safely restore the mdscr and get on with
+	 * handling the syscall.
+	 */
+	regs->pstate |= PSR_D_BIT;
+	return true;
+}
+#else /* CONFIG_ARM64_ERRATUM_1463225 */
+static void cortex_a76_erratum_1463225_svc_handler(void) { }
+static bool cortex_a76_erratum_1463225_debug_handler(struct pt_regs *regs)
+{
+	return false;
+}
+#endif /* CONFIG_ARM64_ERRATUM_1463225 */
+
 static void noinstr el1_abort(struct pt_regs *regs, unsigned long esr)
 {
 	unsigned long far = read_sysreg(far_el1);
@@ -186,7 +235,8 @@ static void noinstr el1_dbg(struct pt_regs *regs, unsigned long esr)
 		gic_write_pmr(GIC_PRIO_IRQON | GIC_PRIO_PSR_I_SET);
 
 	arm64_enter_el1_dbg(regs);
-	do_debug_exception(far, esr, regs);
+	if (!cortex_a76_erratum_1463225_debug_handler(regs))
+		do_debug_exception(far, esr, regs);
 	arm64_exit_el1_dbg(regs);
 }
 
@@ -362,6 +412,7 @@ static void noinstr el0_svc(struct pt_regs *regs)
 		gic_write_pmr(GIC_PRIO_IRQON | GIC_PRIO_PSR_I_SET);
 
 	enter_from_user_mode();
+	cortex_a76_erratum_1463225_svc_handler();
 	do_el0_svc(regs);
 }
 
@@ -439,6 +490,7 @@ static void noinstr el0_svc_compat(struct pt_regs *regs)
 		gic_write_pmr(GIC_PRIO_IRQON | GIC_PRIO_PSR_I_SET);
 
 	enter_from_user_mode();
+	cortex_a76_erratum_1463225_svc_handler();
 	do_el0_svc_compat(regs);
 }