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

[Gerd Hoffmann <kraxel@redhat.com>]

On Thu, Sep 27, 2018 at 04:55:38PM +0100, 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().

Added to vga queue.

cheers,
  Gerd