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From: Peter Maydell <peter.maydell@linaro.org>
To: qemu-devel@nongnu.org
Subject: [PULL 12/14] target/arm: Add sve-default-vector-length cpu property
Date: Tue, 27 Jul 2021 11:47:59 +0100
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From: Richard Henderson <richard.henderson@linaro.org>

Mirror the behavour of /proc/sys/abi/sve_default_vector_length
under the real linux kernel.  We have no way of passing along
a real default across exec like the kernel can, but this is a
decent way of adjusting the startup vector length of a process.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/482
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20210723203344.968563-4-richard.henderson@linaro.org
[PMM: tweaked docs formatting, document -1 special-case,
 added fixup patch from RTH mentioning QEMU's maximum veclen.]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 docs/system/arm/cpu-features.rst | 15 ++++++++
 target/arm/cpu.h                 |  5 +++
 target/arm/cpu.c                 | 14 ++++++--
 target/arm/cpu64.c               | 60 ++++++++++++++++++++++++++++++++
 4 files changed, 92 insertions(+), 2 deletions(-)

diff --git a/docs/system/arm/cpu-features.rst b/docs/system/arm/cpu-feature=
s.rst
index c455442eaf5..11dce5c6037 100644
--- a/docs/system/arm/cpu-features.rst
+++ b/docs/system/arm/cpu-features.rst
@@ -376,3 +376,18 @@ verbose command lines.  However, the recommended way t=
o select vector
 lengths is to explicitly enable each desired length.  Therefore only
 example's (1), (4), and (6) exhibit recommended uses of the properties.
=20
+SVE User-mode Default Vector Length Property
+--------------------------------------------
+
+For qemu-aarch64, the cpu property ``sve-default-vector-length=3DN`` is
+defined to mirror the Linux kernel parameter file
+``/proc/sys/abi/sve_default_vector_length``.  The default length, ``N``,
+is in units of bytes and must be between 16 and 8192.
+If not specified, the default vector length is 64.
+
+If the default length is larger than the maximum vector length enabled,
+the actual vector length will be reduced.  Note that the maximum vector
+length supported by QEMU is 256.
+
+If this property is set to ``-1`` then the default vector length
+is set to the maximum possible length.
diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index be9a4dceae1..9f0a5f84d50 100644
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -1006,6 +1006,11 @@ struct ARMCPU {
     /* Used to set the maximum vector length the cpu will support.  */
     uint32_t sve_max_vq;
=20
+#ifdef CONFIG_USER_ONLY
+    /* Used to set the default vector length at process start. */
+    uint32_t sve_default_vq;
+#endif
+
     /*
      * In sve_vq_map each set bit is a supported vector length of
      * (bit-number + 1) * 16 bytes, i.e. each bit number + 1 is the vector
diff --git a/target/arm/cpu.c b/target/arm/cpu.c
index 752b15bb797..2866dd76588 100644
--- a/target/arm/cpu.c
+++ b/target/arm/cpu.c
@@ -201,7 +201,8 @@ static void arm_cpu_reset(DeviceState *dev)
         env->cp15.cpacr_el1 =3D deposit64(env->cp15.cpacr_el1, 16, 2, 3);
         /* with reasonable vector length */
         if (cpu_isar_feature(aa64_sve, cpu)) {
-            env->vfp.zcr_el[1] =3D MIN(cpu->sve_max_vq - 1, 3);
+            env->vfp.zcr_el[1] =3D
+                aarch64_sve_zcr_get_valid_len(cpu, cpu->sve_default_vq - 1=
);
         }
         /*
          * Enable TBI0 but not TBI1.
@@ -1051,7 +1052,16 @@ static void arm_cpu_initfn(Object *obj)
     QLIST_INIT(&cpu->pre_el_change_hooks);
     QLIST_INIT(&cpu->el_change_hooks);
=20
-#ifndef CONFIG_USER_ONLY
+#ifdef CONFIG_USER_ONLY
+# ifdef TARGET_AARCH64
+    /*
+     * The linux kernel defaults to 512-bit vectors, when sve is supported.
+     * See documentation for /proc/sys/abi/sve_default_vector_length, and
+     * our corresponding sve-default-vector-length cpu property.
+     */
+    cpu->sve_default_vq =3D 4;
+# endif
+#else
     /* Our inbound IRQ and FIQ lines */
     if (kvm_enabled()) {
         /* VIRQ and VFIQ are unused with KVM but we add them to maintain
diff --git a/target/arm/cpu64.c b/target/arm/cpu64.c
index c7a1626bec2..c690318a9b6 100644
--- a/target/arm/cpu64.c
+++ b/target/arm/cpu64.c
@@ -559,6 +559,59 @@ static void cpu_arm_set_sve(Object *obj, bool value, E=
rror **errp)
     cpu->isar.id_aa64pfr0 =3D t;
 }
=20
+#ifdef CONFIG_USER_ONLY
+/* Mirror linux /proc/sys/abi/sve_default_vector_length. */
+static void cpu_arm_set_sve_default_vec_len(Object *obj, Visitor *v,
+                                            const char *name, void *opaque,
+                                            Error **errp)
+{
+    ARMCPU *cpu =3D ARM_CPU(obj);
+    int32_t default_len, default_vq, remainder;
+
+    if (!visit_type_int32(v, name, &default_len, errp)) {
+        return;
+    }
+
+    /* Undocumented, but the kernel allows -1 to indicate "maximum". */
+    if (default_len =3D=3D -1) {
+        cpu->sve_default_vq =3D ARM_MAX_VQ;
+        return;
+    }
+
+    default_vq =3D default_len / 16;
+    remainder =3D default_len % 16;
+
+    /*
+     * Note that the 512 max comes from include/uapi/asm/sve_context.h
+     * and is the maximum architectural width of ZCR_ELx.LEN.
+     */
+    if (remainder || default_vq < 1 || default_vq > 512) {
+        error_setg(errp, "cannot set sve-default-vector-length");
+        if (remainder) {
+            error_append_hint(errp, "Vector length not a multiple of 16\n"=
);
+        } else if (default_vq < 1) {
+            error_append_hint(errp, "Vector length smaller than 16\n");
+        } else {
+            error_append_hint(errp, "Vector length larger than %d\n",
+                              512 * 16);
+        }
+        return;
+    }
+
+    cpu->sve_default_vq =3D default_vq;
+}
+
+static void cpu_arm_get_sve_default_vec_len(Object *obj, Visitor *v,
+                                            const char *name, void *opaque,
+                                            Error **errp)
+{
+    ARMCPU *cpu =3D ARM_CPU(obj);
+    int32_t value =3D cpu->sve_default_vq * 16;
+
+    visit_type_int32(v, name, &value, errp);
+}
+#endif
+
 void aarch64_add_sve_properties(Object *obj)
 {
     uint32_t vq;
@@ -571,6 +624,13 @@ void aarch64_add_sve_properties(Object *obj)
         object_property_add(obj, name, "bool", cpu_arm_get_sve_vq,
                             cpu_arm_set_sve_vq, NULL, NULL);
     }
+
+#ifdef CONFIG_USER_ONLY
+    /* Mirror linux /proc/sys/abi/sve_default_vector_length. */
+    object_property_add(obj, "sve-default-vector-length", "int32",
+                        cpu_arm_get_sve_default_vec_len,
+                        cpu_arm_set_sve_default_vec_len, NULL, NULL);
+#endif
 }
=20
 void arm_cpu_pauth_finalize(ARMCPU *cpu, Error **errp)
--=20
2.20.1