On Thu, Sep 26, 2019 at 01:06:50AM +0100, Vincenzo Frascino wrote: > On 9/25/19 6:08 PM, Catalin Marinas wrote: > > On Wed, Sep 25, 2019 at 09:53:16AM -0700, Nick Desaulniers wrote: > >> On Wed, Sep 25, 2019 at 6:09 AM Catalin Marinas <catalin.mari...@arm.com> > >> wrote: > >>> Suggestions for future improvements of the compat vDSO handling: > >>> > >>> - replace the CROSS_COMPILE_COMPAT prefix with a full COMPATCC; maybe > >>> check that it indeed produces 32-bit code > > CROSS_COMPILE_COMPAT is called like this for symmetry with CROSS_COMPILE.
Actually, what gets in the way is the CONFIG_CROSS_COMPILE_COMPAT_VDSO. We can keep CROSS_COMPILE_COMPAT together with COMPATCC initialised to $(CROSS_COMPILE_COMPAT)gcc. When we will be able to build the compat vDSO with clang, we just pass COMPATCC=clang on the make line and the kernel Makefile will figure out the --target option from CROSS_COMPILE_COMPAT (see how CLANG_FLAGS is handled). If we stick only to env variables or make cmd line (without Kconfig) for the compiler name, we can add a COMPATCC_IS_CLANG in the Kconfig directly and simply not allow the enabling the COMPAT_VDSO if we don't have the right compiler. This saves us warnings during build. > >>> - check whether COMPATCC is clang or not rather than CC_IS_CLANG, which > >>> only checks the native compiler > >> > >> When cross compiling, IIUC CC_IS_CLANG is referring to CC which is the > >> cross compiler, which is different than HOSTCC which is the host > >> compiler. HOSTCC is used mostly for things in scripts/ while CC is > >> used to compile a majority of the kernel in a cross compile. > > > > We need the third compiler here for the compat vDSO (at least with gcc), > > COMPATCC. I'm tempted to just drop the CONFIG_CROSS_COMPILE_COMPAT_VDSO > > altogether and only rely on a COMPATCC. This way we can add > > COMPATCC_IS_CLANG etc. in the Kconfig checks directly. > > > > If clang can build both 32 and 64-bit with the same binary (just > > different options), we could maybe have COMPATCC default to CC and add a > > check on whether COMPATCC generates 32-bit binaries. > > clang requires the --target option for specifying the 32bit triple. > Basically $(TRIPLE)-gcc is equivalent to gcc --target $(TRIPLE). > We need a configuration option to encode this. Since we don't have a CONFIG_* option for the cross-compiler prefix, we shouldn't have one for the compat compiler either. If you want to build the compat vDSO with clang, just pass COMPATCC=clang together with CROSS_COMPILE_COMPAT. We can add Kconfig checks to actually verify that COMPATCC generates 32-bit binaries (e.g. COMPATCC_CAN_LINK32). > >>> - clean up the headers includes; vDSO should not include kernel-only > >>> headers that may even contain code patched at run-time > >> > >> This is a big one; Clang validates the inline asm constraints for > >> extended inline assembly, GCC does not for dead code. So Clang chokes > >> on the inclusion of arm64 headers using extended inline assembly when > >> being compiled for arm-linux-gnueabi. > > > > Whether clang or gcc, I'd like this fixed anyway. At some point we may > > inadvertently rely on some code which is patched at boot time for the > > kernel code but not for the vDSO. > > Do we have any code of this kind in header files? > > The vDSO library uses only a subset of the headers (mainly Macros) hence all > the > unused symbols should be compiled out. Is your concern only theoretical or do > you have an example on where this could be happening? At the moment it's rather theoretical. -- Catalin
