@@ -545,6 +545,7 @@ static void rv64_thead_c906_cpu_init(Object *obj)
cpu->cfg.mvendorid = THEAD_VENDOR_ID;
#ifndef CONFIG_USER_ONLY
set_satp_mode_max_supported(cpu, VM_1_10_SV39);
+ th_register_custom_csrs(cpu);
#endif
/* inherited from parent obj via riscv_cpu_init() */
@@ -824,4 +824,7 @@ void riscv_cpu_register_gdb_regs_for_features(CPUState *cs);
uint8_t satp_mode_max_from_map(uint32_t map);
const char *satp_mode_str(uint8_t satp_mode, bool is_32_bit);
+/* Implemented in th_csr.c */
+void th_register_custom_csrs(RISCVCPU *cpu);
+
#endif /* RISCV_CPU_H */
@@ -33,6 +33,7 @@ riscv_system_ss.add(files(
'monitor.c',
'machine.c',
'pmu.c',
+ 'th_csr.c',
'time_helper.c',
'riscv-qmp-cmds.c',
))
new file mode 100644
@@ -0,0 +1,76 @@
+/*
+ * T-Head-specific CSRs.
+ *
+ * Copyright (c) 2024 VRULL GmbH
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2 or later, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "qemu/osdep.h"
+#include "cpu.h"
+#include "cpu_vendorid.h"
+
+#define CSR_TH_MXSTATUS 0x7c0
+
+/* TH_MXSTATUS bits */
+#define TH_MXSTATUS_UCME BIT(16)
+#define TH_MXSTATUS_MAEE BIT(21)
+#define TH_MXSTATUS_THEADISAEE BIT(22)
+
+typedef struct {
+ int csrno;
+ int (*insertion_test)(RISCVCPU *cpu);
+ riscv_csr_operations csr_ops;
+} riscv_csr;
+
+static RISCVException m_mode(CPURISCVState *env, int csrno)
+{
+ if (env->debugger)
+ return RISCV_EXCP_NONE;
+ if (env->priv != PRV_M)
+ return RISCV_EXCP_ILLEGAL_INST;
+ return RISCV_EXCP_NONE;
+}
+
+static int test_thead_mvendorid(RISCVCPU *cpu)
+{
+ if (cpu->cfg.mvendorid != THEAD_VENDOR_ID)
+ return -1;
+ return 0;
+}
+
+static RISCVException read_th_mxstatus(CPURISCVState *env, int csrno,
+ target_ulong *val)
+{
+ /* We don't set MAEE here, because QEMU does not implement MAEE. */
+ *val = TH_MXSTATUS_UCME | TH_MXSTATUS_THEADISAEE;
+ return RISCV_EXCP_NONE;
+}
+
+static riscv_csr th_csr_list[] = {
+ {
+ .csrno = CSR_TH_MXSTATUS,
+ .insertion_test = test_thead_mvendorid,
+ .csr_ops = { "th.mxstatus", m_mode, read_th_mxstatus }
+ }
+};
+
+void th_register_custom_csrs(RISCVCPU *cpu)
+{
+ for (size_t i = 0; i < ARRAY_SIZE(th_csr_list); i++) {
+ int csrno = th_csr_list[i].csrno;
+ riscv_csr_operations *csr_ops = &th_csr_list[i].csr_ops;
+ if (!th_csr_list[i].insertion_test(cpu))
+ riscv_set_csr_ops(csrno, csr_ops);
+ }
+}
The th.mxstatus CSR can be used to identify available custom extension on T-Head CPUs. The CSR is documented here: https://github.com/T-head-Semi/thead-extension-spec/pull/45 An important property of this patch is, that the th.mxstatus MAEE field is not set (indicating that XTheadMaee is not available). XTheadMaee is a memory attribute extension (similar to Svpbmt) which is implemented in many T-Head CPUs (C906, C910, etc.) and utilizes bits in PTEs that are marked as reserved. QEMU maintainers prefer to not implement XTheadMaee, so we need give kernels a mechanism to identify if XTheadMaee is available in a system or not. And this patch introduces this mechanism in QEMU in a way that's compatible with real HW (i.e., probing the th.mxstatus.MAEE bit). Further context can be found on the list: https://lists.gnu.org/archive/html/qemu-devel/2024-02/msg00775.html Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu> --- target/riscv/cpu.c | 1 + target/riscv/cpu.h | 3 ++ target/riscv/meson.build | 1 + target/riscv/th_csr.c | 76 ++++++++++++++++++++++++++++++++++++++++ 4 files changed, 81 insertions(+) create mode 100644 target/riscv/th_csr.c