[PATCH v4 2/6] target/m68k: Fix fsave/frestore for 68881 FPU

[54weasels <54weasels@gmail.com>]

High level description:
The 68881/68882 FPUs require fsave/frestore instructions to read/write state frames. QEMU only fully implemented this for the 68040. The Sun-3 uses a 68881 FPU, and the boot ROM executes `fsave` to probe FPU presence. This patch adds basic 68881 FPU state frame handling (NULL frame) to accurately simulate an idle FPU state.

Impact on existing functionality:
Fixes FPU detection for 68881/68882 across all M68k boards without affecting 68040 specific state logic.

Context: This patch was originally submitted as part of the monolithic Sun-3 Machine Emulation series (https://patchew.org/QEMU/20260503015756.99176-1-54weasels@gmail.com/) and has been split into atomic components.
---
 target/m68k/translate.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/target/m68k/translate.c b/target/m68k/translate.c
index 138c89d3e5..bdf619883c 100644
--- a/target/m68k/translate.c
+++ b/target/m68k/translate.c
@@ -5364,11 +5364,19 @@ DISAS_INSN(frestore)
         gen_exception(s, s->base.pc_next, EXCP_PRIVILEGE);
         return;
     }
-    if (m68k_feature(s->env, M68K_FEATURE_M68040)) {
+    if (m68k_feature(s->env, M68K_FEATURE_M68040) ||
+        m68k_feature(s->env, M68K_FEATURE_FPU)) {
         SRC_EA(env, addr, OS_LONG, 0, NULL);
-        /* FIXME: check the state frame */
+        if (m68k_feature(s->env, M68K_FEATURE_M68040)) {
+            /* FIXME: check the state frame */
+        } else {
+            /*
+             * 68881/68882 FRESTORE: read the state frame
+             * (NULL frame is 4 bytes)
+             */
+        }
     } else {
-        disas_undef(env, s, insn);
+        disas_undef_fpu(env, s, insn);
     }
 }
 
@@ -5383,8 +5391,12 @@ DISAS_INSN(fsave)
         /* always write IDLE */
         TCGv idle = tcg_constant_i32(0x41000000);
         DEST_EA(env, insn, OS_LONG, idle, NULL);
+    } else if (m68k_feature(s->env, M68K_FEATURE_FPU)) {
+        /* 68881/68882 FSAVE: always write NULL frame */
+        TCGv null_frame = tcg_constant_i32(0x00000000);
+        DEST_EA(env, insn, OS_LONG, null_frame, NULL);
     } else {
-        disas_undef(env, s, insn);
+        disas_undef_fpu(env, s, insn);
     }
 }
 #endif
-- 
2.50.1 (Apple Git-155)