target/riscv/kvm: do not read unavailable CSRs

[1] reports that commit 4db19d5b21 broke a KVM guest running kernel 6.6.
This happens because the kernel does not know 'senvcfg', making it
unable to boot because QEMU is reading/wriiting it without any checks.

After converting the CSRs to do "automated" get/put reg procedures in
the previous patch we can now scan for availability. Two functions are
created:

- kvm_riscv_read_csr_cfg_legacy() will check if the CSR exists by brute
  forcing KVM_GET_ONE_REG in each one of them, interpreting an EINVAL
  return as indication that the CSR isn't available. This will be use in
  absence of KVM_GET_REG_LIST;

- kvm_riscv_read_csr_cfg() will use the existing result of get_reg_list
  to check if the CSRs ids are present.

kvm_riscv_init_multiext_cfg() is now kvm_riscv_init_cfg() to reflect that
the function is also dealing with CSRs.

[1] https://lore.kernel.org/qemu-riscv/CABJz62OfUDHYkQ0T3rGHStQprf1c7_E0qBLbLKhfv=+jb0SYAw@mail.gmail.com/

Fixes: 4db19d5b21 ("target/riscv/kvm: add missing KVM CSRs")
Reported-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250429124421.223883-7-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Cc: qemu-stable@nongnu.org
(cherry picked from commit f396c217a53d9b7960dd002fbb07cfe1d46b27aa)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

target/riscv/kvm/kvm-cpu.c

@@ -636,6 +636,10 @@ static int kvm_riscv_get_regs_csr(CPUState *cs)
     for (i = 0; i < ARRAY_SIZE(kvm_csr_cfgs); i++) {
         KVMCPUConfig *csr_cfg = &kvm_csr_cfgs[i];
 
+        if (!csr_cfg->supported) {
+            continue;
+        }
+
         ret = kvm_get_one_reg(cs, csr_cfg->kvm_reg_id, &reg);
         if (ret) {
             return ret;
@@ -662,6 +666,10 @@ static int kvm_riscv_put_regs_csr(CPUState *cs)
     for (i = 0; i < ARRAY_SIZE(kvm_csr_cfgs); i++) {
         KVMCPUConfig *csr_cfg = &kvm_csr_cfgs[i];
 
+        if (!csr_cfg->supported) {
+            continue;
+        }
+
         if (KVM_REG_SIZE(csr_cfg->kvm_reg_id) == sizeof(uint32_t)) {
             reg = kvm_cpu_csr_get_u32(cpu, csr_cfg);
         } else if (KVM_REG_SIZE(csr_cfg->kvm_reg_id) == sizeof(uint64_t)) {
@@ -1090,6 +1098,32 @@ static void kvm_riscv_read_multiext_legacy(RISCVCPU *cpu,
     }
 }
 
+static void kvm_riscv_read_csr_cfg_legacy(KVMScratchCPU *kvmcpu)
+{
+    uint64_t val;
+    int i, ret;
+
+    for (i = 0; i < ARRAY_SIZE(kvm_csr_cfgs); i++) {
+        KVMCPUConfig *csr_cfg = &kvm_csr_cfgs[i];
+        struct kvm_one_reg reg;
+
+        reg.id = csr_cfg->kvm_reg_id;
+        reg.addr = (uint64_t)&val;
+        ret = ioctl(kvmcpu->cpufd, KVM_GET_ONE_REG, &reg);
+        if (ret != 0) {
+            if (errno == EINVAL) {
+                csr_cfg->supported = false;
+            } else {
+                error_report("Unable to read KVM CSR %s: %s",
+                             csr_cfg->name, strerror(errno));
+                exit(EXIT_FAILURE);
+            }
+        } else {
+            csr_cfg->supported = true;
+        }
+    }
+}
+
 static int uint64_cmp(const void *a, const void *b)
 {
     uint64_t val1 = *(const uint64_t *)a;
@@ -1146,7 +1180,26 @@ static void kvm_riscv_read_vlenb(RISCVCPU *cpu, KVMScratchCPU *kvmcpu,
     }
 }
 
-static void kvm_riscv_init_multiext_cfg(RISCVCPU *cpu, KVMScratchCPU *kvmcpu)
+static void kvm_riscv_read_csr_cfg(struct kvm_reg_list *reglist)
+{
+    struct kvm_reg_list *reg_search;
+    uint64_t reg_id;
+
+    for (int i = 0; i < ARRAY_SIZE(kvm_csr_cfgs); i++) {
+        KVMCPUConfig *csr_cfg = &kvm_csr_cfgs[i];
+
+        reg_id = csr_cfg->kvm_reg_id;
+        reg_search = bsearch(&reg_id, reglist->reg, reglist->n,
+                             sizeof(uint64_t), uint64_cmp);
+        if (!reg_search) {
+            continue;
+        }
+
+        csr_cfg->supported = true;
+    }
+}
+
+static void kvm_riscv_init_cfg(RISCVCPU *cpu, KVMScratchCPU *kvmcpu)
 {
     g_autofree struct kvm_reg_list *reglist = NULL;
     KVMCPUConfig *multi_ext_cfg;
@@ -1163,7 +1216,9 @@ static void kvm_riscv_init_multiext_cfg(RISCVCPU *cpu, KVMScratchCPU *kvmcpu)
      * (EINVAL). Use read_legacy() in this case.
      */
     if (errno == EINVAL) {
-        return kvm_riscv_read_multiext_legacy(cpu, kvmcpu);
+        kvm_riscv_read_multiext_legacy(cpu, kvmcpu);
+        kvm_riscv_read_csr_cfg_legacy(kvmcpu);
+        return;
     } else if (errno != E2BIG) {
         /*
          * E2BIG is an expected error message for the API since we
@@ -1226,6 +1281,7 @@ static void kvm_riscv_init_multiext_cfg(RISCVCPU *cpu, KVMScratchCPU *kvmcpu)
     }
 
     kvm_riscv_check_sbi_dbcn_support(cpu, reglist);
+    kvm_riscv_read_csr_cfg(reglist);
 }
 
 static void riscv_init_kvm_registers(Object *cpu_obj)
@@ -1239,7 +1295,7 @@ static void riscv_init_kvm_registers(Object *cpu_obj)
 
     kvm_riscv_init_machine_ids(cpu, &kvmcpu);
     kvm_riscv_init_misa_ext_mask(cpu, &kvmcpu);
-    kvm_riscv_init_multiext_cfg(cpu, &kvmcpu);
+    kvm_riscv_init_cfg(cpu, &kvmcpu);
 
     kvm_riscv_destroy_scratch_vcpu(&kvmcpu);
 }