hw/display/qxl: Suppress clang-7 warning about misaligned atomic operation

If QEMU is compiled with clang-7 it results in the warning:

hw/display/qxl.c:1884:19: error: misaligned or large atomic operation
may incur significant performance penalty [-Werror,-Watomic-alignment]
    old_pending = atomic_fetch_or(&d->ram->int_pending, le_events);
                  ^

This is because the Spice headers forgot to define the QXLRam struct
with the '__aligned__(4)' attribute.  clang 7 and newer will thus
warn that the access here to int_pending might not be 4-aligned
(because the QXLRam object d->ram points at might start at a
misaligned address).  In fact we set up d->ram in init_qxl_ram() so
it always starts at a 4K boundary, so we know the atomic access here
is OK.

Newer Spice versions (with Spice commit
beda5ec7a6848be20c0cac2a9a8ef2a41e8069c1) will fix the bug;
for older Spice versions, work around it by telling the compiler
explicitly that the alignment is OK using __builtin_assume_aligned().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180927155538.699-1-peter.maydell@linaro.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

1 files changed, 25 insertions, 1 deletions

diff --git a/hw/display/qxl.c b/hw/display/qxl.c
index 7479864..e628cf1 100644
--- a/hw/display/qxl.c
+++ b/hw/display/qxl.c
@@ -1893,7 +1893,31 @@ static void qxl_send_events(PCIQXLDevice *d, uint32_t events)
         trace_qxl_send_events_vm_stopped(d->id, events);
         return;
     }
-    old_pending = atomic_fetch_or(&d->ram->int_pending, le_events);
+    /*
+     * Older versions of Spice forgot to define the QXLRam struct
+     * with the '__aligned__(4)' attribute. clang 7 and newer will
+     * thus warn that atomic_fetch_or(&d->ram->int_pending, ...)
+     * might be a misaligned atomic access, and will generate an
+     * out-of-line call for it, which results in a link error since
+     * we don't currently link against libatomic.
+     *
+     * In fact we set up d->ram in init_qxl_ram() so it always starts
+     * at a 4K boundary, so we know that &d->ram->int_pending is
+     * naturally aligned for a uint32_t. Newer Spice versions
+     * (with Spice commit beda5ec7a6848be20c0cac2a9a8ef2a41e8069c1)
+     * will fix the bug directly. To deal with older versions,
+     * we tell the compiler to assume the address really is aligned.
+     * Any compiler which cares about the misalignment will have
+     * __builtin_assume_aligned.
+     */
+#ifdef HAS_ASSUME_ALIGNED
+#define ALIGNED_UINT32_PTR(P) ((uint32_t *)__builtin_assume_aligned(P, 4))
+#else
+#define ALIGNED_UINT32_PTR(P) ((uint32_t *)P)
+#endif
+
+    old_pending = atomic_fetch_or(ALIGNED_UINT32_PTR(&d->ram->int_pending),
+                                  le_events);
     if ((old_pending & le_events) == le_events) {
         return;
     }