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>
---
 target/riscv/kvm/kvm-cpu.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/target/riscv/kvm/kvm-cpu.c b/target/riscv/kvm/kvm-cpu.c
index 45b6cf1cfa..273c71baea 100644
--- a/target/riscv/kvm/kvm-cpu.c
+++ b/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);
@@ -1481,11 +1485,36 @@ 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 = {
-- 
2.41.0

On Fri, Dec 1, 2023 at 5:00 AM Daniel Henrique Barboza
<dbarboza@ventanamicro.com> wrote:
>
> 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>

Alistair

> ---
>  target/riscv/kvm/kvm-cpu.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
>
> diff --git a/target/riscv/kvm/kvm-cpu.c b/target/riscv/kvm/kvm-cpu.c
> index 45b6cf1cfa..273c71baea 100644
> --- a/target/riscv/kvm/kvm-cpu.c
> +++ b/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);
> @@ -1481,11 +1485,36 @@ 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 = {
> --
> 2.41.0
>
>