linux-kernel - [PATCH 2/2] MIPS: ebpf jit: Implement R4000 workarounds

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Date:   Wed, 22 Feb 2023 16:12:22 +0000
From:   Jiaxun Yang <jiaxun.yang@...goat.com>
To:     linux-mips@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, tsbogend@...ha.franken.de,
        johan.almbladh@...finetworks.com, paulburton@...nel.org,
        bpf@...r.kernel.org, Jiaxun Yang <jiaxun.yang@...goat.com>
Subject: [PATCH 2/2] MIPS: ebpf jit: Implement R4000 workarounds

For R4000 erratas around multiplication and division instructions,
as our use of those instructions are always followed by mflo/mfhi
instructions, the only issue we need care is

"MIPS R4000PC/SC Errata, Processor Revision 2.2 and 3.0" Errata 28:
"A double-word or a variable shift may give an incorrect result if
executed while an integer multiplication is in progress."

We just emit a mfhi $0 to ensure the operation is completed after
every multiplication instruction accorading to workaround suggestion
in the document.

Signed-off-by: Jiaxun Yang <jiaxun.yang@...goat.com>
---
 arch/mips/Kconfig              | 4 +---
 arch/mips/net/bpf_jit_comp32.c | 4 ++++
 arch/mips/net/bpf_jit_comp64.c | 3 +++
 3 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index df0910e3895c..5ea07c833c5b 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -63,9 +63,7 @@ config MIPS
 	select HAVE_DEBUG_STACKOVERFLOW
 	select HAVE_DMA_CONTIGUOUS
 	select HAVE_DYNAMIC_FTRACE
-	select HAVE_EBPF_JIT if !CPU_MICROMIPS && \
-				!CPU_R4000_WORKAROUNDS && \
-				!CPU_R4400_WORKAROUNDS
+	select HAVE_EBPF_JIT if !CPU_MICROMIPS
 	select HAVE_EXIT_THREAD
 	select HAVE_FAST_GUP
 	select HAVE_FTRACE_MCOUNT_RECORD
diff --git a/arch/mips/net/bpf_jit_comp32.c b/arch/mips/net/bpf_jit_comp32.c
index ace5db3fbd17..fee334544d2f 100644
--- a/arch/mips/net/bpf_jit_comp32.c
+++ b/arch/mips/net/bpf_jit_comp32.c
@@ -446,6 +446,9 @@ static void emit_mul_i64(struct jit_context *ctx, const u8 dst[], s32 imm)
 		} else {
 			emit(ctx, multu, hi(dst), src);
 			emit(ctx, mflo, hi(dst));
+			/* Ensure multiplication is completed */
+			if (IS_ENABLED(CONFIG_CPU_R4000_WORKAROUNDS))
+				emit(ctx, mfhi, MIPS_R_ZERO);
 		}
 
 		/* hi(dst) = hi(dst) - lo(dst) */
@@ -504,6 +507,7 @@ static void emit_mul_r64(struct jit_context *ctx,
 	} else {
 		emit(ctx, multu, lo(dst), lo(src));
 		emit(ctx, mflo, lo(dst));
+		/* No need for workaround because we have this mfhi */
 		emit(ctx, mfhi, tmp);
 	}
 
diff --git a/arch/mips/net/bpf_jit_comp64.c b/arch/mips/net/bpf_jit_comp64.c
index 0e7c1bdcf914..5f5a93f997bc 100644
--- a/arch/mips/net/bpf_jit_comp64.c
+++ b/arch/mips/net/bpf_jit_comp64.c
@@ -228,6 +228,9 @@ static void emit_alu_r64(struct jit_context *ctx, u8 dst, u8 src, u8 op)
 		} else {
 			emit(ctx, dmultu, dst, src);
 			emit(ctx, mflo, dst);
+			/* Ensure multiplication is completed */
+			if (IS_ENABLED(CONFIG_CPU_R4000_WORKAROUNDS))
+				emit(ctx, mfhi, MIPS_R_ZERO);
 		}
 		break;
 	/* dst = dst / src */
-- 
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