[PATCH v3 0/3] OpenRISC updates for user space FPU

[PATCH v3 0/3] OpenRISC updates for user space FPU

[Stafford Horne]

Hello,

Since v2:
 - Add reviewed-by's from Richard
 - Pull cpu definition out of ifdef in helper_mfspr
Since v1:
 - Fixups suggested by Richard Henderson

This series adds support for the FPU related architecture changes defined in
architecture spec revision v1.4.

 - https://openrisc.io/revisions/r1.4

In summary the architecture changes are:

 - Change FPCSR SPR permissions to allow for reading and writing from user
   space.
 - Clarify that FPU underflow detection is done by detecting tininess before
   rounding.

Previous to this series FPCSR reads and writes from user-mode in QEMU would
throw an illegal argument exception.  The proper behavior should have been to
treat these operations as no-ops as the cpu implementations do.  As mentioned
series changes FPCSR read/write to follow the spec.

The series has been tested with the FPU support added in glibc test suite and
all math tests are passing.


Stafford Horne (3):
  target/openrisc: Allow fpcsr access in user mode
  target/openrisc: Set PC to cpu state on FPU exception
  target/openrisc: Setup FPU for detecting tininess before rounding

 target/openrisc/cpu.c        |  4 ++
 target/openrisc/fpu_helper.c | 13 ++++++-
 target/openrisc/sys_helper.c | 45 ++++++++++++++++------
 target/openrisc/translate.c  | 72 ++++++++++++++++--------------------
 4 files changed, 81 insertions(+), 53 deletions(-)

-- 
2.39.1

[PATCH v3 1/3] target/openrisc: Allow fpcsr access in user mode

[Stafford Horne]

As per OpenRISC spec 1.4 FPCSR can be read and written in user mode.

Update mtspr and mfspr helpers to support this by moving the is_user
check into the helper.

Link: https://raw.githubusercontent.com/openrisc/doc/master/openrisc-arch-1.4-rev0.pdf
Signed-off-by: Stafford Horne <shorne@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
---

Since v2:
 - Add reviewed-by
 - In helper_mfspr bring cpu out of ifdef to avoid replicatig the definition.
   Originally I left it in the ifdef to avoid having to mix having pointers and
   the data array defined on the stack.  But that's overthinking.
Since v1:
 - Update commit message to remove text about no-existant logic change.

 target/openrisc/sys_helper.c | 45 ++++++++++++++++------
 target/openrisc/translate.c  | 72 ++++++++++++++++--------------------
 2 files changed, 66 insertions(+), 51 deletions(-)

diff --git a/target/openrisc/sys_helper.c b/target/openrisc/sys_helper.c
index ec145960e3..ccdee3b8be 100644
--- a/target/openrisc/sys_helper.c
+++ b/target/openrisc/sys_helper.c
@@ -29,17 +29,37 @@
 
 #define TO_SPR(group, number) (((group) << 11) + (number))
 
+static inline bool is_user(CPUOpenRISCState *env)
+{
+#ifdef CONFIG_USER_ONLY
+    return true;
+#else
+    return (env->sr & SR_SM) == 0;
+#endif
+}
+
 void HELPER(mtspr)(CPUOpenRISCState *env, target_ulong spr, target_ulong rb)
 {
-#ifndef CONFIG_USER_ONLY
     OpenRISCCPU *cpu = env_archcpu(env);
+#ifndef CONFIG_USER_ONLY
     CPUState *cs = env_cpu(env);
     target_ulong mr;
     int idx;
 #endif
 
+    /* Handle user accessible SPRs first.  */
     switch (spr) {
+    case TO_SPR(0, 20): /* FPCSR */
+        cpu_set_fpcsr(env, rb);
+        return;
+    }
+
+    if (is_user(env)) {
+        raise_exception(cpu, EXCP_ILLEGAL);
+    }
+
 #ifndef CONFIG_USER_ONLY
+    switch (spr) {
     case TO_SPR(0, 11): /* EVBAR */
         env->evbar = rb;
         break;
@@ -187,27 +207,33 @@ void HELPER(mtspr)(CPUOpenRISCState *env, target_ulong spr, target_ulong rb)
         cpu_openrisc_timer_update(cpu);
         qemu_mutex_unlock_iothread();
         break;
-#endif
-
-    case TO_SPR(0, 20): /* FPCSR */
-        cpu_set_fpcsr(env, rb);
-        break;
     }
+#endif
 }
 
 target_ulong HELPER(mfspr)(CPUOpenRISCState *env, target_ulong rd,
                            target_ulong spr)
 {
+    OpenRISCCPU *cpu = env_archcpu(env);
 #ifndef CONFIG_USER_ONLY
     uint64_t data[TARGET_INSN_START_WORDS];
     MachineState *ms = MACHINE(qdev_get_machine());
-    OpenRISCCPU *cpu = env_archcpu(env);
     CPUState *cs = env_cpu(env);
     int idx;
 #endif
 
+    /* Handle user accessible SPRs first.  */
     switch (spr) {
+    case TO_SPR(0, 20): /* FPCSR */
+        return env->fpcsr;
+    }
+
+    if (is_user(env)) {
+        raise_exception(cpu, EXCP_ILLEGAL);
+    }
+
 #ifndef CONFIG_USER_ONLY
+    switch (spr) {
     case TO_SPR(0, 0): /* VR */
         return env->vr;
 
@@ -324,11 +350,8 @@ target_ulong HELPER(mfspr)(CPUOpenRISCState *env, target_ulong rd,
         cpu_openrisc_count_update(cpu);
         qemu_mutex_unlock_iothread();
         return cpu_openrisc_count_get(cpu);
-#endif
-
-    case TO_SPR(0, 20): /* FPCSR */
-        return env->fpcsr;
     }
+#endif
 
     /* for rd is passed in, if rd unchanged, just keep it back.  */
     return rd;
diff --git a/target/openrisc/translate.c b/target/openrisc/translate.c
index 76e53c78d4..43ba0cc1ad 100644
--- a/target/openrisc/translate.c
+++ b/target/openrisc/translate.c
@@ -819,45 +819,12 @@ static bool trans_l_xori(DisasContext *dc, arg_rri *a)
 
 static bool trans_l_mfspr(DisasContext *dc, arg_l_mfspr *a)
 {
-    check_r0_write(dc, a->d);
-
-    if (is_user(dc)) {
-        gen_illegal_exception(dc);
-    } else {
-        TCGv spr = tcg_temp_new();
-
-        if (tb_cflags(dc->base.tb) & CF_USE_ICOUNT) {
-            gen_io_start();
-            if (dc->delayed_branch) {
-                tcg_gen_mov_tl(cpu_pc, jmp_pc);
-                tcg_gen_discard_tl(jmp_pc);
-            } else {
-                tcg_gen_movi_tl(cpu_pc, dc->base.pc_next + 4);
-            }
-            dc->base.is_jmp = DISAS_EXIT;
-        }
+    TCGv spr = tcg_temp_new();
 
-        tcg_gen_ori_tl(spr, cpu_R(dc, a->a), a->k);
-        gen_helper_mfspr(cpu_R(dc, a->d), cpu_env, cpu_R(dc, a->d), spr);
-    }
-    return true;
-}
-
-static bool trans_l_mtspr(DisasContext *dc, arg_l_mtspr *a)
-{
-    if (is_user(dc)) {
-        gen_illegal_exception(dc);
-    } else {
-        TCGv spr;
+    check_r0_write(dc, a->d);
 
-        if (tb_cflags(dc->base.tb) & CF_USE_ICOUNT) {
-            gen_io_start();
-        }
-        /* For SR, we will need to exit the TB to recognize the new
-         * exception state.  For NPC, in theory this counts as a branch
-         * (although the SPR only exists for use by an ICE).  Save all
-         * of the cpu state first, allowing it to be overwritten.
-         */
+    if (tb_cflags(dc->base.tb) & CF_USE_ICOUNT) {
+        gen_io_start();
         if (dc->delayed_branch) {
             tcg_gen_mov_tl(cpu_pc, jmp_pc);
             tcg_gen_discard_tl(jmp_pc);
@@ -865,11 +832,36 @@ static bool trans_l_mtspr(DisasContext *dc, arg_l_mtspr *a)
             tcg_gen_movi_tl(cpu_pc, dc->base.pc_next + 4);
         }
         dc->base.is_jmp = DISAS_EXIT;
+    }
+
+    tcg_gen_ori_tl(spr, cpu_R(dc, a->a), a->k);
+    gen_helper_mfspr(cpu_R(dc, a->d), cpu_env, cpu_R(dc, a->d), spr);
+    return true;
+}
+
+static bool trans_l_mtspr(DisasContext *dc, arg_l_mtspr *a)
+{
+    TCGv spr = tcg_temp_new();
 
-        spr = tcg_temp_new();
-        tcg_gen_ori_tl(spr, cpu_R(dc, a->a), a->k);
-        gen_helper_mtspr(cpu_env, spr, cpu_R(dc, a->b));
+    if (tb_cflags(dc->base.tb) & CF_USE_ICOUNT) {
+        gen_io_start();
     }
+    /*
+     * For SR, we will need to exit the TB to recognize the new
+     * exception state.  For NPC, in theory this counts as a branch
+     * (although the SPR only exists for use by an ICE).  Save all
+     * of the cpu state first, allowing it to be overwritten.
+     */
+    if (dc->delayed_branch) {
+        tcg_gen_mov_tl(cpu_pc, jmp_pc);
+        tcg_gen_discard_tl(jmp_pc);
+    } else {
+        tcg_gen_movi_tl(cpu_pc, dc->base.pc_next + 4);
+    }
+    dc->base.is_jmp = DISAS_EXIT;
+
+    tcg_gen_ori_tl(spr, cpu_R(dc, a->a), a->k);
+    gen_helper_mtspr(cpu_env, spr, cpu_R(dc, a->b));
     return true;
 }
 
-- 
2.39.1

[PATCH v3 2/3] target/openrisc: Set PC to cpu state on FPU exception

[Stafford Horne]

Store the PC to ensure the correct value can be read in the exception
handler.

Signed-off-by: Stafford Horne <shorne@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
---
Since v2:
 - Add reviewed-by
Since v1:
 - Use function do_fpe (similar to do_range) to raise exception.

 target/openrisc/fpu_helper.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/target/openrisc/fpu_helper.c b/target/openrisc/fpu_helper.c
index f9e34fa2cc..8b81d2f62f 100644
--- a/target/openrisc/fpu_helper.c
+++ b/target/openrisc/fpu_helper.c
@@ -20,8 +20,8 @@
 
 #include "qemu/osdep.h"
 #include "cpu.h"
+#include "exec/exec-all.h"
 #include "exec/helper-proto.h"
-#include "exception.h"
 #include "fpu/softfloat.h"
 
 static int ieee_ex_to_openrisc(int fexcp)
@@ -45,6 +45,15 @@ static int ieee_ex_to_openrisc(int fexcp)
     return ret;
 }
 
+static G_NORETURN
+void do_fpe(CPUOpenRISCState *env, uintptr_t pc)
+{
+    CPUState *cs = env_cpu(env);
+
+    cs->exception_index = EXCP_FPE;
+    cpu_loop_exit_restore(cs, pc);
+}
+
 void HELPER(update_fpcsr)(CPUOpenRISCState *env)
 {
     int tmp = get_float_exception_flags(&env->fp_status);
@@ -55,7 +64,7 @@ void HELPER(update_fpcsr)(CPUOpenRISCState *env)
         if (tmp) {
             env->fpcsr |= tmp;
             if (env->fpcsr & FPCSR_FPEE) {
-                helper_exception(env, EXCP_FPE);
+                do_fpe(env, GETPC());
             }
         }
     }
-- 
2.39.1

[PATCH v3 3/3] target/openrisc: Setup FPU for detecting tininess before rounding

[Stafford Horne]

OpenRISC defines tininess to be detected before rounding.  Setup qemu to
obey this.

Signed-off-by: Stafford Horne <shorne@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
---
Since v2:
 - Add reviewed-by
Since v1:
 - Remove setting default NaN behavior.

 target/openrisc/cpu.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/target/openrisc/cpu.c b/target/openrisc/cpu.c
index 0ce4f796fa..61d748cfdc 100644
--- a/target/openrisc/cpu.c
+++ b/target/openrisc/cpu.c
@@ -22,6 +22,7 @@
 #include "qemu/qemu-print.h"
 #include "cpu.h"
 #include "exec/exec-all.h"
+#include "fpu/softfloat-helpers.h"
 #include "tcg/tcg.h"
 
 static void openrisc_cpu_set_pc(CPUState *cs, vaddr value)
@@ -90,6 +91,9 @@ static void openrisc_cpu_reset_hold(Object *obj)
     s->exception_index = -1;
     cpu_set_fpcsr(&cpu->env, 0);
 
+    set_float_detect_tininess(float_tininess_before_rounding,
+                              &cpu->env.fp_status);
+
 #ifndef CONFIG_USER_ONLY
     cpu->env.picmr = 0x00000000;
     cpu->env.picsr = 0x00000000;
-- 
2.39.1