target/riscv/kvm: do PR_RISCV_V_SET_CONTROL during realize()

Linux RISC-V vector documentation (Document/arch/riscv/vector.rst)
mandates a prctl() in order to allow an userspace thread to use the
Vector extension from the host.

This is something to be done in realize() time, after init(), when we
already decided whether we're using RVV or not. We don't have a
realize() callback for KVM yet, so add kvm_cpu_realize() and enable RVV
for the thread via PR_RISCV_V_SET_CONTROL.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20231218204321.75757-4-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/kvm/kvm-cpu.c

@@ -18,6 +18,7 @@
 
 #include "qemu/osdep.h"
 #include <sys/ioctl.h>
+#include <sys/prctl.h>
 
 #include <linux/kvm.h>
 
@@ -47,6 +48,9 @@
 #include "sysemu/runstate.h"
 #include "hw/riscv/numa.h"
 
+#define PR_RISCV_V_SET_CONTROL            69
+#define PR_RISCV_V_VSTATE_CTRL_ON          2
+
 void riscv_kvm_aplic_request(void *opaque, int irq, int level)
 {
     kvm_set_irq(kvm_state, irq, !!level);
@@ -1496,11 +1500,36 @@ static void kvm_cpu_instance_init(CPUState *cs)
     }
 }
 
+/*
+ * We'll get here via the following path:
+ *
+ * riscv_cpu_realize()
+ *   -> cpu_exec_realizefn()
+ *      -> kvm_cpu_realize() (via accel_cpu_common_realize())
+ */
+static bool kvm_cpu_realize(CPUState *cs, Error **errp)
+{
+    RISCVCPU *cpu = RISCV_CPU(cs);
+    int ret;
+
+    if (riscv_has_ext(&cpu->env, RVV)) {
+        ret = prctl(PR_RISCV_V_SET_CONTROL, PR_RISCV_V_VSTATE_CTRL_ON);
+        if (ret) {
+            error_setg(errp, "Error in prctl PR_RISCV_V_SET_CONTROL, code: %s",
+                       strerrorname_np(errno));
+            return false;
+        }
+    }
+
+   return true;
+}
+
 static void kvm_cpu_accel_class_init(ObjectClass *oc, void *data)
 {
     AccelCPUClass *acc = ACCEL_CPU_CLASS(oc);
 
     acc->cpu_instance_init = kvm_cpu_instance_init;
+    acc->cpu_target_realize = kvm_cpu_realize;
 }
 
 static const TypeInfo kvm_cpu_accel_type_info = {