accel/tcg: Avoid reading too much in load_atom_{2,4}

When load_atom_extract_al16_or_al8 is inexpensive, we want to use
it early, in order to avoid the overhead of required_atomicity.
However, we must not read past the end of the page.

If there are more than 8 bytes remaining, then both the "aligned 16"
and "aligned 8" paths align down so that the read has at least
16 bytes remaining on the page.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

1 files changed, 8 insertions, 2 deletions

diff --git a/accel/tcg/ldst_atomicity.c.inc b/accel/tcg/ldst_atomicity.c.inc
index e5c590a499..1b793e6935 100644
--- a/accel/tcg/ldst_atomicity.c.inc
+++ b/accel/tcg/ldst_atomicity.c.inc
@@ -404,7 +404,10 @@ static uint16_t load_atom_2(CPUArchState *env, uintptr_t ra,
         return load_atomic2(pv);
     }
     if (HAVE_ATOMIC128_RO) {
-        return load_atom_extract_al16_or_al8(pv, 2);
+        intptr_t left_in_page = -(pi | TARGET_PAGE_MASK);
+        if (likely(left_in_page > 8)) {
+            return load_atom_extract_al16_or_al8(pv, 2);
+        }
     }
 
     atmax = required_atomicity(env, pi, memop);
@@ -443,7 +446,10 @@ static uint32_t load_atom_4(CPUArchState *env, uintptr_t ra,
         return load_atomic4(pv);
     }
     if (HAVE_ATOMIC128_RO) {
-        return load_atom_extract_al16_or_al8(pv, 4);
+        intptr_t left_in_page = -(pi | TARGET_PAGE_MASK);
+        if (likely(left_in_page > 8)) {
+            return load_atom_extract_al16_or_al8(pv, 4);
+        }
     }
 
     atmax = required_atomicity(env, pi, memop);