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Thu, 02 Jul 2026 23:52:00 -0700 (PDT) From: Danielle Lam X-Google-Original-From: Danielle Lam To: qemu-devel@nongnu.org Cc: qemu-riscv@nongnu.org, palmer@dabbelt.com, alistair.francis@wdc.com, liwei1518@gmail.com, daniel.barboza@oss.qualcomm.com, zhiwei_liu@linux.alibaba.com, Danielle Lam Subject: [PATCH] target/riscv: fix amocas with rd = x0 followed by beq involving x0 is not taken when it should be Date: Fri, 3 Jul 2026 13:51:52 +0700 Message-ID: <20260703065152.23024-1-danlam@g.hmc.edu> X-Mailer: git-send-email 2.54.0 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists1p.gnu.org; Received-SPF: pass client-ip=2607:f8b0:4864:20::62c; envelope-from=phuong.lamthanhthao@gmail.com; helo=mail-pl1-x62c.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-Mailman-Approved-At: Fri, 03 Jul 2026 07:52:12 -0400 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @gmail.com) X-ZM-MESSAGEID: 1783079594624158500 Content-Type: text/plain; charset="utf-8" amocas.{w,d,q,b,h} implement compare and swap by reusing rd current value a= s both comparator input and as the output that tcg_gen_atomic_cmpxchg_* wri= tes the old memory into. Before the fix, ret =3D=3D cmpv and we name that v= ariable dest. When rd =3D x0, dest was ctx->zero. Then since ret =3D=3D cmp= v in the first place, the atomic op overwrites ret with ctx->zero which is = not the same zero value. Any later instructions that refer back to ret for = the initial zero value which is now changed, resulting in 0xbaaaaaad0000000= 0. The fix is to use different variables for ret and cmpv and assign that to a= tomic op to run. **TESTING** gen_cmpxchg .d was tested and it passed. .w/.b/.h use the same logic so the fix was sim= ilar. gen_cmpxchg64 .d was not tested trans_amocas_q .q had its own custom test: test_amocasq.S .section .text .globl _start _start: la t0, scratch li t1, 0x1111 li t2, 0x2222 sd t1, 0(t0) sd t2, 8(t0) li a0, 0x3333 li a1, 0x4444 amocas.q x0, a0, (t0) addi a2, x0, 0 spin: j spin .section .bss .align 4 scratch: .space 16 Then searching the trace log: grep -n "x12/a2" amocasq-test.trace.log I found that all matches read zero. 10612696: x12/a2 0 x13/a3 0 x14/a4 0 x15/a5 0 ... 10613892: x12/a2 0 x13/a3 0 x14/a4 0 x15/a5 0 Run against the unpatched code, a2 read back as 0x2222. This confirms the t= est run as intended and the fix works. Signed-off-by: Danielle Lam --- target/riscv/insn_trans/trans_rvzacas.c.inc | 13 ++++++++----- target/riscv/translate.c | 5 +++-- 2 files changed, 11 insertions(+), 7 deletions(-) diff --git a/target/riscv/insn_trans/trans_rvzacas.c.inc b/target/riscv/ins= n_trans/trans_rvzacas.c.inc index 79bca1e957..867f5e5ade 100644 --- a/target/riscv/insn_trans/trans_rvzacas.c.inc +++ b/target/riscv/insn_trans/trans_rvzacas.c.inc @@ -72,13 +72,14 @@ static bool gen_cmpxchg64(DisasContext *ctx, arg_atomic= *a, MemOp mop) return false; } =20 - TCGv_i64 dest =3D get_gpr_pair(ctx, a->rd); TCGv src1 =3D get_address(ctx, a->rs1, 0); TCGv_i64 src2 =3D get_gpr_pair(ctx, a->rs2); + TCGv_i64 cmpv =3D get_gpr_pair(ctx, a->rd); + TCGv_i64 dest =3D tcg_temp_new_i64(); =20 mop |=3D ctx->mo_endianness; decode_save_opc(ctx, RISCV_UW2_ALWAYS_STORE_AMO); - tcg_gen_atomic_cmpxchg_i64(dest, src1, dest, src2, ctx->mem_idx, mop); + tcg_gen_atomic_cmpxchg_i64(dest, src1, cmpv, src2, ctx->mem_idx, mop); =20 gen_set_gpr_pair(ctx, a->rd, dest); return true; @@ -117,13 +118,15 @@ static bool trans_amocas_q(DisasContext *ctx, arg_amo= cas_q *a) TCGv_i128 src2 =3D tcg_temp_new_i128(); TCGv_i64 src2l =3D get_gpr(ctx, a->rs2, EXT_NONE); TCGv_i64 src2h =3D get_gpr(ctx, a->rs2 =3D=3D 0 ? 0 : a->rs2 + 1, EXT_= NONE); - TCGv_i64 destl =3D get_gpr(ctx, a->rd, EXT_NONE); - TCGv_i64 desth =3D get_gpr(ctx, a->rd =3D=3D 0 ? 0 : a->rd + 1, EXT_NO= NE); + TCGv_i64 cmpl =3D get_gpr(ctx, a->rd, EXT_NONE); + TCGv_i64 cmph =3D get_gpr(ctx, a->rd =3D=3D 0 ? 0 : a->rd + 1, EXT_NON= E); + TCGv_i64 destl =3D tcg_temp_new_i64(); + TCGv_i64 desth =3D tcg_temp_new_i64(); MemOp memop =3D MO_ALIGN | MO_UO; =20 memop |=3D ctx->mo_endianness; tcg_gen_concat_i64_i128(src2, src2l, src2h); - tcg_gen_concat_i64_i128(dest, destl, desth); + tcg_gen_concat_i64_i128(dest, cmpl, cmph); decode_save_opc(ctx, RISCV_UW2_ALWAYS_STORE_AMO); tcg_gen_atomic_cmpxchg_i128(dest, src1, dest, src2, ctx->mem_idx, memo= p); =20 diff --git a/target/riscv/translate.c b/target/riscv/translate.c index 9684dbe752..62a9af9b56 100644 --- a/target/riscv/translate.c +++ b/target/riscv/translate.c @@ -1166,13 +1166,14 @@ static bool gen_amo(DisasContext *ctx, arg_atomic *= a, =20 static bool gen_cmpxchg(DisasContext *ctx, arg_atomic *a, MemOp mop) { - TCGv dest =3D get_gpr(ctx, a->rd, EXT_NONE); TCGv src1 =3D get_address(ctx, a->rs1, 0); TCGv src2 =3D get_gpr(ctx, a->rs2, EXT_NONE); + TCGv cmpv =3D get_gpr(ctx, a->rd, EXT_NONE); + TCGv dest =3D dest_gpr(ctx, a->rd); =20 mop |=3D ctx->mo_endianness; decode_save_opc(ctx, RISCV_UW2_ALWAYS_STORE_AMO); - tcg_gen_atomic_cmpxchg_tl(dest, src1, dest, src2, ctx->mem_idx, mop); + tcg_gen_atomic_cmpxchg_tl(dest, src1, cmpv, src2, ctx->mem_idx, mop); =20 gen_set_gpr(ctx, a->rd, dest); return true; --=20 2.47.3