[PATCH v2] riscv: thead: Add th.sxstatus CSR emulation

["Christoph Müllner" <christoph.muellner@vrull.eu>]

The th.sxstatus 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/46

An important property of this patch is, that the th.sxstatus 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.sxstatus.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    | 78 ++++++++++++++++++++++++++++++++++++++++
 4 files changed, 83 insertions(+)
 create mode 100644 target/riscv/th_csr.c

diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
index 36e3e5fdaf..b82ba95ae6 100644
--- a/target/riscv/cpu.c
+++ b/target/riscv/cpu.c
@@ -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() */
diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
index 3b1a02b944..c9f8f06751 100644
--- a/target/riscv/cpu.h
+++ b/target/riscv/cpu.h
@@ -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 */
diff --git a/target/riscv/meson.build b/target/riscv/meson.build
index a5e0734e7f..a4bd61e52a 100644
--- a/target/riscv/meson.build
+++ b/target/riscv/meson.build
@@ -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',
 ))
diff --git a/target/riscv/th_csr.c b/target/riscv/th_csr.c
new file mode 100644
index 0000000000..66d260cabd
--- /dev/null
+++ b/target/riscv/th_csr.c
@@ -0,0 +1,78 @@
+/*
+ * 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_SXSTATUS 0x5c0
+
+/* TH_SXSTATUS bits */
+#define TH_SXSTATUS_UCME        BIT(16)
+#define TH_SXSTATUS_MAEE        BIT(21)
+#define TH_SXSTATUS_THEADISAEE  BIT(22)
+
+typedef struct {
+    int csrno;
+    int (*insertion_test)(RISCVCPU *cpu);
+    riscv_csr_operations csr_ops;
+} riscv_csr;
+
+static RISCVException s_mode_csr(CPURISCVState *env, int csrno)
+{
+    if (env->debugger)
+        return RISCV_EXCP_NONE;
+
+    if (env->priv >= PRV_S)
+        return RISCV_EXCP_NONE;
+
+    return RISCV_EXCP_ILLEGAL_INST;
+}
+
+static int test_thead_mvendorid(RISCVCPU *cpu)
+{
+    if (cpu->cfg.mvendorid != THEAD_VENDOR_ID)
+        return -1;
+    return 0;
+}
+
+static RISCVException read_th_sxstatus(CPURISCVState *env, int csrno,
+                                       target_ulong *val)
+{
+    /* We don't set MAEE here, because QEMU does not implement MAEE. */
+    *val = TH_SXSTATUS_UCME | TH_SXSTATUS_THEADISAEE;
+    return RISCV_EXCP_NONE;
+}
+
+static riscv_csr th_csr_list[] = {
+    {
+        .csrno = CSR_TH_SXSTATUS,
+        .insertion_test = test_thead_mvendorid,
+        .csr_ops = { "th.sxstatus", s_mode_csr, read_th_sxstatus }
+    }
+};
+
+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);
+    }
+}
-- 
2.44.0