tree 6f76bd0576c31d849f31cb960de0e0c7af7a5573
parent 2bb15bddf2607110820d5ce5aa43baac27292fb3
author Paolo Bonzini <pbonzini@redhat.com> 1470387097 +0200
committer Paolo Bonzini <pbonzini@redhat.com> 1470776256 +0200

linuxboot_dma: avoid guest ABI breakage on gcc vs. clang compilation

Recent GCC compiles linuxboot_dma.c to 921 bytes, while CentOS 6 needs
1029 and clang needs 1527.  Because the size of the ROM, rounded to the
next 512 bytes, must match, this causes the API to break between a <1K
ROM and one that is bigger.

We want to make the ROM 1.5 KB in size, but it's better to make clang
produce leaner ROMs, because currently it is worryingly close to the limit.
To fix this prevent clang's happy inlining (which -Os cannot prevent).
This only requires adding a noinline attribute.

Second, the patch makes sure that the ROM has enough padding to prevent
ABI breakage on different compilers.  The size is now hardcoded in the file
that is passed to signrom.py, as was the case before commit 6f71b77
("scripts/signrom.py: Allow option ROM checksum script to write the size
header.", 2016-05-23); signrom.py however will still pad the input to
the requested size.  This ensures that the padding goes beyond the
next multiple of 512 if necessary, and also avoids the need for
-fno-toplevel-reorder which clang doesn't support.  signrom.py can then
error out if the requested size is too small for the actual size of the
compiled ROM.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>