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X-Received-From: 2607:f8b0:400e:c05::244 Subject: [Qemu-devel] [PULL 13/20] RISC-V: Hardwire satp to 0 for no-mmu case X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Sagar Karandikar , Bastian Koppelmann , Palmer Dabbelt , Michael Clark , Alistair Francis , patches@groups.riscv.org Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail-DKIM: fail (Header signature does not verify) X-ZohoMail: RDKM_2 RSF_0 Z_629925259 SPT_0 Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" satp is WARL so it should not trap on illegal writes, rather it can be hardwired to zero and silently ignore illegal writes. It seems the RISC-V WARL behaviour is preferred to having to trap overhead versus simply reading back the value and checking if the write took (saves hundreds of cycles and more complex trap handling code). Cc: Sagar Karandikar Cc: Bastian Koppelmann Cc: Palmer Dabbelt Cc: Alistair Francis Signed-off-by: Michael Clark Reviewed-by: Alistair Francis --- target/riscv/op_helper.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/target/riscv/op_helper.c b/target/riscv/op_helper.c index 7c6068bac958..101dac1ee8dc 100644 --- a/target/riscv/op_helper.c +++ b/target/riscv/op_helper.c @@ -255,7 +255,7 @@ void csr_write_helper(CPURISCVState *env, target_ulong = val_to_write, } case CSR_SATP: /* CSR_SPTBR */ { if (!riscv_feature(env, RISCV_FEATURE_MMU)) { - goto do_illegal; + break; } if (env->priv_ver <=3D PRIV_VERSION_1_09_1 && (val_to_write ^ env-= >sptbr)) { @@ -465,7 +465,10 @@ target_ulong csr_read_helper(CPURISCVState *env, targe= t_ulong csrno) return env->scounteren; case CSR_SCAUSE: return env->scause; - case CSR_SPTBR: + case CSR_SATP: /* CSR_SPTBR */ + if (!riscv_feature(env, RISCV_FEATURE_MMU)) { + return 0; + } if (env->priv_ver >=3D PRIV_VERSION_1_10_0) { return env->satp; } else { --=20 2.7.0