linux/arch/um
Linus Torvalds 4356e9f841 work around gcc bugs with 'asm goto' with outputs
We've had issues with gcc and 'asm goto' before, and we created a
'asm_volatile_goto()' macro for that in the past: see commits
3f0116c323 ("compiler/gcc4: Add quirk for 'asm goto' miscompilation
bug") and a9f180345f ("compiler/gcc4: Make quirk for
asm_volatile_goto() unconditional").

Then, much later, we ended up removing the workaround in commit
43c249ea0b ("compiler-gcc.h: remove ancient workaround for gcc PR
58670") because we no longer supported building the kernel with the
affected gcc versions, but we left the macro uses around.

Now, Sean Christopherson reports a new version of a very similar
problem, which is fixed by re-applying that ancient workaround.  But the
problem in question is limited to only the 'asm goto with outputs'
cases, so instead of re-introducing the old workaround as-is, let's
rename and limit the workaround to just that much less common case.

It looks like there are at least two separate issues that all hit in
this area:

 (a) some versions of gcc don't mark the asm goto as 'volatile' when it
     has outputs:

        https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98619
        https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110420

     which is easy to work around by just adding the 'volatile' by hand.

 (b) Internal compiler errors:

        https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110422

     which are worked around by adding the extra empty 'asm' as a
     barrier, as in the original workaround.

but the problem Sean sees may be a third thing since it involves bad
code generation (not an ICE) even with the manually added 'volatile'.

but the same old workaround works for this case, even if this feels a
bit like voodoo programming and may only be hiding the issue.

Reported-and-tested-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/all/20240208220604.140859-1-seanjc@google.com/
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: Jakub Jelinek <jakub@redhat.com>
Cc: Andrew Pinski <quic_apinski@quicinc.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2024-02-09 15:57:48 -08:00
..
configs um: Fix hostaudio build errors 2023-08-19 23:30:22 +02:00
drivers TTY/Serial changes for 6.8-rc1 2024-01-18 11:37:24 -08:00
include work around gcc bugs with 'asm goto' with outputs 2024-02-09 15:57:48 -08:00
kernel This pull request contains the following changes for UML: 2024-01-17 10:44:34 -08:00
os-Linux um: Remove unused register save/restore functions 2024-01-05 16:28:57 +01:00
scripts um: prevent user code in modules 2023-04-20 23:06:38 +02:00
.gitignore um: gitignore: Add kernel/capflags.c 2021-12-21 21:22:43 +01:00
Kbuild um: use obj-y to descend into arch/um/*/ 2023-08-26 22:41:34 +02:00
Kconfig Landlock updates for v6.5-rc1 2023-06-27 17:10:27 -07:00
Kconfig.debug um: Disable CONFIG_GCOV with MODULES 2021-04-15 23:10:33 +02:00
Makefile um: Fix adding '-no-pie' for clang 2024-01-28 01:13:37 +09:00
Makefile-os-Linux
Makefile-skas arch: um: Add Clang coverage support 2024-01-05 00:30:35 +01:00