linux-kernel - [PATCH v2 3/3] xtensa: improve call0 ABI probing

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Date:   Fri, 13 May 2022 20:34:37 -0700
From:   Max Filippov <jcmvbkbc@...il.com>
To:     linux-xtensa@...ux-xtensa.org
Cc:     Chris Zankel <chris@...kel.net>, linux-kernel@...r.kernel.org,
        Max Filippov <jcmvbkbc@...il.com>
Subject: [PATCH v2 3/3] xtensa: improve call0 ABI probing

When call0 userspace ABI support by probing is enabled instructions that
cause illegal instruction exception when PS.WOE is clear are retried
with PS.WOE set before calling c-level exception handler. Record user pc
at which PS.WOE was set in the fast exception handler and clear PS.WOE
in the c-level exception handler if we get there from the same address.

Signed-off-by: Max Filippov <jcmvbkbc@...il.com>
---
Changes v1->v2:

- split ABI probing improvement from the artificial division by 0

 arch/xtensa/include/asm/thread_info.h |  4 ++++
 arch/xtensa/kernel/asm-offsets.c      |  3 +++
 arch/xtensa/kernel/entry.S            |  5 +++++
 arch/xtensa/kernel/traps.c            | 12 ++++++++++++
 4 files changed, 24 insertions(+)

diff --git a/arch/xtensa/include/asm/thread_info.h b/arch/xtensa/include/asm/thread_info.h
index 52974317a6b6..326db1c1d5d8 100644
--- a/arch/xtensa/include/asm/thread_info.h
+++ b/arch/xtensa/include/asm/thread_info.h
@@ -56,6 +56,10 @@ struct thread_info {
 	/* result of the most recent exclusive store */
 	unsigned long		atomctl8;
 #endif
+#ifdef CONFIG_USER_ABI_CALL0_PROBE
+	/* Address where PS.WOE was enabled by the ABI probing code */
+	unsigned long		ps_woe_fix_addr;
+#endif
 
 	/*
 	 * If i-th bit is set then coprocessor state is loaded into the
diff --git a/arch/xtensa/kernel/asm-offsets.c b/arch/xtensa/kernel/asm-offsets.c
index 9a1db6ffcbf4..da38de20ae59 100644
--- a/arch/xtensa/kernel/asm-offsets.c
+++ b/arch/xtensa/kernel/asm-offsets.c
@@ -88,6 +88,9 @@ int main(void)
 	OFFSET(TI_STSTUS, thread_info, status);
 	OFFSET(TI_CPU, thread_info, cpu);
 	OFFSET(TI_PRE_COUNT, thread_info, preempt_count);
+#ifdef CONFIG_USER_ABI_CALL0_PROBE
+	OFFSET(TI_PS_WOE_FIX_ADDR, thread_info, ps_woe_fix_addr);
+#endif
 
 	/* struct thread_info (offset from start_struct) */
 	DEFINE(THREAD_RA, offsetof (struct task_struct, thread.ra));
diff --git a/arch/xtensa/kernel/entry.S b/arch/xtensa/kernel/entry.S
index 3224b4ceca34..e3eae648ba2e 100644
--- a/arch/xtensa/kernel/entry.S
+++ b/arch/xtensa/kernel/entry.S
@@ -1056,6 +1056,11 @@ ENTRY(fast_illegal_instruction_user)
 	movi	a3, PS_WOE_MASK
 	or	a0, a0, a3
 	wsr	a0, ps
+#ifdef CONFIG_USER_ABI_CALL0_PROBE
+	GET_THREAD_INFO(a3, a2)
+	rsr	a0, epc1
+	s32i	a0, a3, TI_PS_WOE_FIX_ADDR
+#endif
 	l32i	a3, a2, PT_AREG3
 	l32i	a0, a2, PT_AREG0
 	rsr	a2, depc
diff --git a/arch/xtensa/kernel/traps.c b/arch/xtensa/kernel/traps.c
index 47445b2d4217..21c2c30baf69 100644
--- a/arch/xtensa/kernel/traps.c
+++ b/arch/xtensa/kernel/traps.c
@@ -324,6 +324,18 @@ static int check_div0(struct pt_regs *regs)
 
 static void do_illegal_instruction(struct pt_regs *regs)
 {
+#ifdef CONFIG_USER_ABI_CALL0_PROBE
+	/*
+	 * When call0 application encounters an illegal instruction fast
+	 * exception handler will attempt to set PS.WOE and retry failing
+	 * instruction.
+	 * If we get here we know that that instruction is also illegal
+	 * with PS.WOE set, so it's not related to the windowed option
+	 * hence PS.WOE may be cleared.
+	 */
+	if (regs->pc == current_thread_info()->ps_woe_fix_addr)
+		regs->ps &= ~PS_WOE_MASK;
+#endif
 	if (check_div0(regs)) {
 		do_div0(regs);
 		return;
-- 
2.30.2