On 05/22/2014 01:19 AM, Ian Munsie wrote:

Hi Cody,

I just tried building this with gcc 4.5, which failed with the following
warning (treated as an error):

cc1: warnings being treated as errors
arch/powerpc/perf/hv-24x7.c: In function 'single_24x7_request':
arch/powerpc/perf/hv-24x7.c:346:1: error: the frame size of 8192 bytes is larger than 2048 bytes
make[3]: *** [arch/powerpc/perf/hv-24x7.o] Error 1
make[2]: *** [arch/powerpc/perf] Error 2

My .config has CONFIG_FRAME_WARN=2048 (default on 64bit), but the
alignment constraints in this function may require 8K on the stack -
possibly a bit large?

Yep, it is a bit large. In other places in hv-24x7 that use similar 
firmware interfaces (with similar alignment requirements), I've used a 
kmem_cache (hv_page_cache). Testing out a patch that uses that here as well.

Notably for some reason this warning no longer seems to trigger on gcc
4.8 (or at least somewhere between 4.5-4.8), though the assembly does
still show it aligning the buffers.

That's a bit concerning (and might be why I didn't pick it up, using gcc 
4.9.0 over here). Looking at the gcc docs, it seems to indicate that 
alloca() and VLAs aren't counted for -Wframe-larger-than. Perhaps gcc 
decided to move locally defined structures with alignment requirements 
into that same bucket? (while size of the structures is statically 
determinable, the stack consumption due to alignment is [to some degree] 
variable).