| Message ID | 1462812590-4494-2-git-send-email-andre.przywara@arm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Mon, May 09, 2016 at 05:49:45PM +0100, Andre Przywara wrote: > Commit 77ee306c0aea9 ("arm64: alternatives: add enable parameter to > conditional asm macros") extended the alternative assembly macros. > Unfortunately this does not really work as one would expect, as the > enable parameter in fact correctly protects the alternative section > magic, but not the actual code sequences. I don't remember how we ended up with this code because it's not used anywhere. > So if enable is false, we will have the original instruction(s) _and_ > the alternative ones in the file, which is just wrong. > To make this work one would need to additionally protect the > alternative sequence with extra .if directives, which makes > the intention of the enable parameter rather pointless. > Instead users should directly guard the whole "_else; insn; _endif" > sequence with .if directives. > Add a comment describing this usage and drop the enable parameter from > the alternative_else and alternative_endif macros. > > This reverts parts of commit 77ee306c0aea ("arm64: alternatives: add > enable parameter to conditional asm macros"). > > Signed-off-by: Andre Przywara <andre.przywara@arm.com> > --- > arch/arm64/include/asm/alternative.h | 24 ++++++++++++++++++------ > 1 file changed, 18 insertions(+), 6 deletions(-) > > diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h > index beccbde..502c9ef 100644 > --- a/arch/arm64/include/asm/alternative.h > +++ b/arch/arm64/include/asm/alternative.h > @@ -94,6 +94,8 @@ void apply_alternatives(void *start, size_t length); > * > * The code that follows this macro will be assembled and linked as > * normal. There are no restrictions on this code. > + * If you use the enable parameter, see the comments below for _else > + * and _endif. > */ > .macro alternative_if_not cap, enable = 1 > .if \enable > @@ -117,23 +119,33 @@ void apply_alternatives(void *start, size_t length); > * 2. Not contain a branch target that is used outside of the > * alternative sequence it is defined in (branches into an > * alternative sequence are not fixed up). > + * > + * If you used the optional enable parameter in the opening > + * alternative_if_not macro above, please protect the whole _else > + * branch with an .if directive: > + * alternative_if_not CAP_SOMETHING, condition > + * orig_insn > + * .if condition > + * alternative_else > + * repl_insn > + * alternative_endif > + * .endif I think the intention was to that in the !condition case, both alternatives are dropped. That includes the original code. I propose we revert the original commit, I don't think it is strictly needed for your patches (I'll try to fix them up and see how it goes).
diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h index beccbde..502c9ef 100644 --- a/arch/arm64/include/asm/alternative.h +++ b/arch/arm64/include/asm/alternative.h @@ -94,6 +94,8 @@ void apply_alternatives(void *start, size_t length); * * The code that follows this macro will be assembled and linked as * normal. There are no restrictions on this code. + * If you use the enable parameter, see the comments below for _else + * and _endif. */ .macro alternative_if_not cap, enable = 1 .if \enable @@ -117,23 +119,33 @@ void apply_alternatives(void *start, size_t length); * 2. Not contain a branch target that is used outside of the * alternative sequence it is defined in (branches into an * alternative sequence are not fixed up). + * + * If you used the optional enable parameter in the opening + * alternative_if_not macro above, please protect the whole _else + * branch with an .if directive: + * alternative_if_not CAP_SOMETHING, condition + * orig_insn + * .if condition + * alternative_else + * repl_insn + * alternative_endif + * .endif */ -.macro alternative_else, enable = 1 - .if \enable +.macro alternative_else 662: .pushsection .altinstr_replacement, "ax" 663: - .endif .endm /* * Complete an alternative code sequence. + * + * Please mind the comment at alternative_else above if you used the + * optional enable parameter with the opening alternative_if_not macro. */ -.macro alternative_endif, enable = 1 - .if \enable +.macro alternative_endif 664: .popsection .org . - (664b-663b) + (662b-661b) .org . - (662b-661b) + (664b-663b) - .endif .endm #define _ALTERNATIVE_CFG(insn1, insn2, cap, cfg, ...) \
Commit 77ee306c0aea9 ("arm64: alternatives: add enable parameter to conditional asm macros") extended the alternative assembly macros. Unfortunately this does not really work as one would expect, as the enable parameter in fact correctly protects the alternative section magic, but not the actual code sequences. So if enable is false, we will have the original instruction(s) _and_ the alternative ones in the file, which is just wrong. To make this work one would need to additionally protect the alternative sequence with extra .if directives, which makes the intention of the enable parameter rather pointless. Instead users should directly guard the whole "_else; insn; _endif" sequence with .if directives. Add a comment describing this usage and drop the enable parameter from the alternative_else and alternative_endif macros. This reverts parts of commit 77ee306c0aea ("arm64: alternatives: add enable parameter to conditional asm macros"). Signed-off-by: Andre Przywara <andre.przywara@arm.com> --- arch/arm64/include/asm/alternative.h | 24 ++++++++++++++++++------ 1 file changed, 18 insertions(+), 6 deletions(-)