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

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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>
This commit is contained in:
Peter Maydell 2018-09-27 16:55:38 +01:00 committed by Gerd Hoffmann
parent 1242429b20
commit 5a358b39f5
2 changed files with 34 additions and 1 deletions
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26
hw/display/qxl.c
View File

@ -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;
}

9
include/qemu/compiler.h
View File

@ -122,6 +122,15 @@
#ifndef __has_feature
#define __has_feature(x) 0 /* compatibility with non-clang compilers */
#endif
#ifndef __has_builtin
#define __has_builtin(x) 0 /* compatibility with non-clang compilers */
#endif
#if __has_builtin(__builtin_assume_aligned) || QEMU_GNUC_PREREQ(4, 7)
#define HAS_ASSUME_ALIGNED
#endif
/* Implement C11 _Generic via GCC builtins. Example:
*
* QEMU_GENERIC(x, (float, sinf), (long double, sinl), sin) (x)