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Re: [Qemu-devel] [PATCH] hw/display/qxl: Suppress clang-7 warning about

From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH] hw/display/qxl: Suppress clang-7 warning about misaligned atomic operation
Date: Thu, 27 Sep 2018 10:10:07 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0

On 9/27/18 8:55 AM, Peter Maydell wrote:
> 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 <address@hidden>
> ---
>  include/qemu/compiler.h |  9 +++++++++
>  hw/display/qxl.c        | 26 +++++++++++++++++++++++++-
>  2 files changed, 34 insertions(+), 1 deletion(-)

Reviewed-by: Richard Henderson <address@hidden>


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