accel/tcg: Perform aligned atomic reads in translator_ld

Perform aligned atomic reads in translator_ld, if possible.
According to

https://lore.kernel.org/qemu-devel/20240607101403.1109-1-jim.shu@sifive.com/

this is required for RISC-V Ziccif.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

1 files changed, 38 insertions, 4 deletions

diff --git a/accel/tcg/translator.c b/accel/tcg/translator.c
index ef1538b..157be33 100644
--- a/accel/tcg/translator.c
+++ b/accel/tcg/translator.c
@@ -265,12 +265,14 @@ static bool translator_ld(CPUArchState *env, DisasContextBase *db,
 
     if (likely(((base ^ last) & TARGET_PAGE_MASK) == 0)) {
         /* Entire read is from the first page. */
-        memcpy(dest, host + (pc - base), len);
-        return true;
+        goto do_read;
     }
 
     if (unlikely(((base ^ pc) & TARGET_PAGE_MASK) == 0)) {
-        /* Read begins on the first page and extends to the second. */
+        /*
+         * Read begins on the first page and extends to the second.
+         * The unaligned read is never atomic.
+         */
         size_t len0 = -(pc | TARGET_PAGE_MASK);
         memcpy(dest, host + (pc - base), len0);
         pc += len0;
@@ -329,7 +331,39 @@ static bool translator_ld(CPUArchState *env, DisasContextBase *db,
         host = db->host_addr[1];
     }
 
-    memcpy(dest, host + (pc - base), len);
+ do_read:
+    /*
+     * Assume aligned reads should be atomic, if possible.
+     * We're not in a position to jump out with EXCP_ATOMIC.
+     */
+    host += pc - base;
+    switch (len) {
+    case 2:
+        if (QEMU_IS_ALIGNED(pc, 2)) {
+            uint16_t t = qatomic_read((uint16_t *)host);
+            stw_he_p(dest, t);
+            return true;
+        }
+        break;
+    case 4:
+        if (QEMU_IS_ALIGNED(pc, 4)) {
+            uint32_t t = qatomic_read((uint32_t *)host);
+            stl_he_p(dest, t);
+            return true;
+        }
+        break;
+#ifdef CONFIG_ATOMIC64
+    case 8:
+        if (QEMU_IS_ALIGNED(pc, 8)) {
+            uint64_t t = qatomic_read__nocheck((uint64_t *)host);
+            stq_he_p(dest, t);
+            return true;
+        }
+        break;
+#endif
+    }
+    /* Unaligned or partial read from the second page is not atomic. */
+    memcpy(dest, host, len);
     return true;
 }