This is a note to let you know that I've just added the patch titled Kconfig: Add option for asm goto w/ tied outputs to workaround clang-13 bug to the 5.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: kconfig-add-option-for-asm-goto-w-tied-outputs-to-workaround-clang-13-bug.patch and it can be found in the queue-5.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 1aa0e8b144b6474c4914439d232d15bfe883636b Mon Sep 17 00:00:00 2001 From: Sean Christopherson <seanjc@xxxxxxxxxx> Date: Wed, 2 Feb 2022 00:49:41 +0000 Subject: Kconfig: Add option for asm goto w/ tied outputs to workaround clang-13 bug From: Sean Christopherson <seanjc@xxxxxxxxxx> commit 1aa0e8b144b6474c4914439d232d15bfe883636b upstream. Add a config option to guard (future) usage of asm_volatile_goto() that includes "tied outputs", i.e. "+" constraints that specify both an input and output parameter. clang-13 has a bug[1] that causes compilation of such inline asm to fail, and KVM wants to use a "+m" constraint to implement a uaccess form of CMPXCHG[2]. E.g. the test code fails with <stdin>:1:29: error: invalid operand in inline asm: '.long (${1:l}) - .' int foo(int *x) { asm goto (".long (%l[bar]) - .\n": "+m"(*x) ::: bar); return *x; bar: return 0; } ^ <stdin>:1:29: error: unknown token in expression <inline asm>:1:9: note: instantiated into assembly here .long () - . ^ 2 errors generated. on clang-13, but passes on gcc (with appropriate asm goto support). The bug is fixed in clang-14, but won't be backported to clang-13 as the changes are too invasive/risky. gcc also had a similar bug[3], fixed in gcc-11, where gcc failed to account for its behavior of assigning two numbers to tied outputs (one for input, one for output) when evaluating symbolic references. [1] https://github.com/ClangBuiltLinux/linux/issues/1512 [2] https://lore.kernel.org/all/YfMruK8%2F1izZ2VHS@xxxxxxxxxx [3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98096 Suggested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> Message-Id: <20220202004945.2540433-2-seanjc@xxxxxxxxxx> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- init/Kconfig | 5 +++++ 1 file changed, 5 insertions(+) --- a/init/Kconfig +++ b/init/Kconfig @@ -33,6 +33,11 @@ config CC_CAN_LINK config CC_HAS_ASM_GOTO def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC)) +config CC_HAS_ASM_GOTO_TIED_OUTPUT + depends on CC_HAS_ASM_GOTO_OUTPUT + # Detect buggy gcc and clang, fixed in gcc-11 clang-14. + def_bool $(success,echo 'int foo(int *x) { asm goto (".long (%l[bar]) - .\n": "+m"(*x) ::: bar); return *x; bar: return 0; }' | $CC -x c - -c -o /dev/null) + config TOOLS_SUPPORT_RELR def_bool $(success,env "CC=$(CC)" "LD=$(LD)" "NM=$(NM)" "OBJCOPY=$(OBJCOPY)" $(srctree)/scripts/tools-support-relr.sh) Patches currently in stable-queue which might be from seanjc@xxxxxxxxxx are queue-5.4/kconfig-add-option-for-asm-goto-w-tied-outputs-to-workaround-clang-13-bug.patch queue-5.4/kvm-nvmx-leave-most-vm-exit-info-fields-unmodified-o.patch