<caoj.f...@cn.fujitsu.com>,Greg KH <gre...@linuxfoundation.org>,jarkko.sakki...@linux.intel.com,jgr...@suse.com,Josh Poimboeuf <jpoim...@redhat.com>,Matthias Kaehlcke <m...@chromium.org>,thomas.lenda...@amd.com,Thiebaud Weksteen <tw...@google.com>,mj...@google.com,j...@perches.com From: h...@zytor.com Message-ID: <191e4ebe-4cb2-4c8b-ab61-689a91ffe...@zytor.com>
On June 12, 2018 11:33:14 AM PDT, Nick Desaulniers <ndesaulni...@google.com> wrote: >On Fri, Jun 8, 2018 at 3:04 AM Sedat Dilek <sedat.di...@gmail.com> >wrote: >> >> On Fri, Jun 8, 2018 at 9:59 AM, Arnd Bergmann <a...@arndb.de> wrote: >> > On Thu, Jun 7, 2018 at 10:49 PM, Nick Desaulniers >> > <ndesaulni...@google.com> wrote: >> >> Functions marked extern inline do not emit an externally visible >> >> function when the gnu89 C standard is used. Some KBUILD Makefiles >> >> overwrite KBUILD_CFLAGS. This is an issue for GCC 5.1+ users as >without >> >> an explicit C standard specified, the default is gnu11. Since c99, >the >> >> semantics of extern inline have changed such that an externally >visible >> >> function is always emitted. This can lead to multiple definition >errors >> >> of extern inline functions at link time of compilation units whose >build >> >> files have removed an explicit C standard compiler flag for users >of GCC >> >> 5.1+ or Clang. >> >> >> >> Signed-off-by: Nick Desaulniers <ndesaulni...@google.com> >> >> Suggested-by: H. Peter Anvin <h...@zytor.com> >> >> Suggested-by: Joe Perches <j...@perches.com> >> > >> > I suspect this will break Geert's gcc-4.1.2, which I think doesn't >have that >> > attribute yet (4.1.3 or higher have it according to the >documentation. >> > >> > It wouldn't be hard to work around that if we want to keep that >version >> > working, or we could decide that it's time to officially stop >supporting >> > that version, but we should probably decide on one or the other. > >Heh, so earlier we decided against compiler flags (-std=gnu89 or >-fgnu89-inline) in preference to function attributes. The function >attribute is preferable as some of the Makefiles [accidentally?] >overwrite KBUILD_CFLAGS, which is problematic for gcc 5.1 users as the >implicit c standard used was changed to gnu11 from gnu89. What's nice >is that to support gcc 4.1 users, we simply don't need to add any >attribute, as their implicit c standard is gnu89 which has the >semantics for extern inline that we want. I have a simple change to >this patch that can support users of various gcc versions, see below: > >> Good point. >> What is the minimum requirement of GCC version currently? >> AFAICS x86/asm-goto support requires GCC >= 4.5? > >Yes, but that's only for x86, IIUC. It seems the kernel may have >different minimum required versions of GCC based on arch then? That >may be ok, but I'm not sure that's easy to keep track of without >having it explicitly stated somewhere like the docs perhaps? > >> Just FYI... >> ...saw the last days in upstream commits that kbuild/kconfig for >> 4.18-rc1 offers possibilities to check for cc-version dependencies. > >Those will be helpful. If we want to pursue compiler flags, which get >set some Makefiles, then yes. But I think a simpler change to my >patch would be as below. > >It seems gcc did not get __has_attribute [0] until 5.1, but will >define __GNUC_GNU_INLINE__ if supported. [1] Unfortunately, Clang >does not define __GNUC_GNU_INLINE__ [2]. So a proper feature test >might look like: > >``` >#ifndef __has_attribute >#define __has_attribute(x) 0 >#endif > >#if defined(__GNUC_GNU_INLINE__) || __has_attribute(gnu_inline) >#define __gnu_inline __attribute__(gnu_inline) >#endif > >#define inline inline __attribute__((always_inline, unused)) notrace >__gnu_inline >``` > >Thoughts on this approach? I can send a v5 tomorrow if there's no >major issues. Feedback appreciated, as always. > >[0] https://clang.llvm.org/docs/LanguageExtensions.html#has-attribute >[1] >https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes >[2] https://bugs.llvm.org/show_bug.cgi?id=37784 Please fix clang. It isn't all that hard to fix. However, __GCC_GNU_INLINE__ means you are in GNU mode by default, on gcc's new enough to have multiple misses. The right thing to look for is __GCC_STDC_INLINE__ in which case you need the attribute. By the way, you should check clang against gcc's predefined macros by doing: gcc [options] -x c -Wp,-dM -E /dev/null | sort Options can change the predefined macros substantially, especially the, -std=, arch and -O options. -x c can be replaced with e.g. -x c++, objective-c, assembler-with-cpp etc. -- Sent from my Android device with K-9 Mail. Please excuse my brevity. _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
