Messages in this thread | | | Date | Fri, 12 Feb 2021 09:40:59 -0500 | From | Steven Rostedt <> | Subject | Re: [PATCH] RFC: x86/jump_label: Mark arguments as const to satisfy asm constraints |
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On Thu, 11 Feb 2021 13:48:48 -0800 Jason Gerecke <killertofu@gmail.com> wrote:
> When compiling an external kernel module with `-O0` or `-O1`, the following > compile error may be reported: > > ./arch/x86/include/asm/jump_label.h:25:2: error: impossible constraint in ‘asm’ > 25 | asm_volatile_goto("1:" > | ^~~~~~~~~~~~~~~~~ > > It appears that these lower optimization levels prevent GCC from detecting > that the key/branch arguments can be treated as constants and used as > immediate operands. To work around this, explicitly add the `const` label.
Yes this makes sense. The "i" constraint needs to be a constant.
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
-- Steve
> > Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com> > Cc: Steven Rostedt <rostedt@goodmis.org> > Cc: Ard Biesheuvel <ardb@kernel.org> > --- > Marked RFC since I'm not familiar with this subsystem or the asm blocks that > are impacted. Extra-close inspection would be appreciated. > > arch/x86/include/asm/jump_label.h | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h > index 06c3cc22a058..7f2006645d84 100644 > --- a/arch/x86/include/asm/jump_label.h > +++ b/arch/x86/include/asm/jump_label.h > @@ -20,7 +20,7 @@ > #include <linux/stringify.h> > #include <linux/types.h> > > -static __always_inline bool arch_static_branch(struct static_key *key, bool branch) > +static __always_inline bool arch_static_branch(struct static_key * const key, const bool branch) > { > asm_volatile_goto("1:" > ".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t" > @@ -36,7 +36,7 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran > return true; > } > > -static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch) > +static __always_inline bool arch_static_branch_jump(struct static_key * const key, const bool branch) > { > asm_volatile_goto("1:" > ".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"
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