On 28.05.2020 20:10, Andrew Cooper wrote: > On 28/05/2020 11:25, Jan Beulich wrote: >> On 27.05.2020 21:18, Andrew Cooper wrote: >>> --- a/xen/arch/x86/Kconfig >>> +++ b/xen/arch/x86/Kconfig >>> @@ -34,6 +34,10 @@ config ARCH_DEFCONFIG >>> config INDIRECT_THUNK >>> def_bool $(cc-option,-mindirect-branch-register) >>> >>> +config HAS_AS_CET >>> + # binutils >= 2.29 and LLVM >= 7 >>> + def_bool $(as-instr,wrssq %rax$(comma)0;setssbsy;endbr64) >> So you put me in a really awkward position: I'd really like to see >> this series go in for 4.14, yet I've previously indicated I want the >> underlying concept to first be agreed upon, before any uses get >> introduced. > > There are already users. One of them is even in context.
Hmm, indeed. I clearly didn't notice this aspect when reviewing Anthony's series. > I don't see that there is anything open for dispute in the first place. > Being able to do exactly this was a one key driving factor to a newer > Kconfig, because it is superior mechanism to the ad-hoc mess we had > previously (not to mention, a vast detriment to build time). This "key driving factor" was presumably from your perspective. Could you point me to a discussion (and resulting decision) that this is an explicit goal of that work? I don't recall any, and hence I also don't recall having been given a chance in influence the direction, decision, and overall outcome. In the interest of getting this series in for 4.14, and on the assumption that you're willing to have a discussion on the direction wrt storing tool chain capabilities in .config before any further uses get added (and with the potential need to undo the ones we have / gain here) Reviewed-by: Jan Beulich <jbeul...@suse.com> The comment at the very end may point at an issue that wants sorting, the others below are merely replies to some of the points you've made. >> A fundamental problem with this, at least as long as (a) more of >> Anthony's series hasn't been committed and (b) we re-build Xen upon >> installing (as root), even if it was fully built (as non-root) and >> is ready without any re-building. > > How is any any of this relevant to Anthony's recent changes? Command line options not getting written to .*.cmd and read back and compare upon rebuild is the issue here. Albeit thinking about it again for anything that's stored in .config that's for now compensated for by a change to .config triggering a global rebuild. The specific issue I had run into was with XEN_BUILD_EFI changing without xen.lds getting re-generated. > You've always been asking for trouble if your `make` before `make > install` has as different toolchain, even in regular autotools/userspace > software. The newer Kconfig logic might make this trouble far more > obvious, but doesn't introduce the problem. I disagree. There should be no dependency on the tool chain at all at install time, with a fully built tree. It ought to be fine to not even have a tool chain accessible to root, even less so the precise one that was used for building the tree. >>> --- a/xen/scripts/Kconfig.include >>> +++ b/xen/scripts/Kconfig.include >>> @@ -31,6 +31,10 @@ cc-option = $(success,$(CC) -Werror $(CLANG_FLAGS) $(1) >>> -E -x c /dev/null -o /de >>> # Return y if the linker supports <flag>, n otherwise >>> ld-option = $(success,$(LD) -v $(1)) >>> >>> +# $(as-instr,<instr>) >>> +# Return y if the assembler supports <instr>, n otherwise >>> +as-instr = $(success,printf "%b\n" "$(1)" | $(CC) $(CLANG_FLAGS) -c -x >>> assembler -o /dev/null -) >> Is this actually checking the right thing in the clang case? > > Appears to work correctly for me. (Again, its either fine, or need > bugfixing anyway for 4.14, and raising with Linux as this is unmodified > upstream code like the rest of Kconfig.include). This answer made me try it out: On a system with clang 5 and binutils 2.32 "incsspd %eax" translates fine with -no-integrated-as but doesn't without. The previously mentioned odd use of CLANG_FLAGS, perhaps together with the disconnect from where we establish whether to use -no-integrated-as in the first place (arch.mk; I have no idea whether the CFLAGS determined would be usable by the kconfig invocation, nor how to solve the chicken-and-egg problem there if this is meant to work that way), may be the reason for this. Cc-ing Anthony once again ... As an aside - this being taken from Linux doesn't mean it's suitable for our use. For example, Linux'es way to use (or not) -no-integrated-as is entirely different from ours (via LLVM_IAS setting, used in the top level Makefile). I don't think I can see whether the check above, for the case at hand, would work correctly there, both with and without that variable set. Jan
