Messages in this thread | | | Date | Sat, 4 Nov 2023 09:34:42 +0000 | Subject | Re: Missing clobber on alternative use on Linux UM 32-bit | From | Anton Ivanov <> |
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On 04/11/2023 09:25, Nadav Amit wrote: > I was reading (again) the x86 C macro of “alternative()” and I was a bit > surprised it does clobber the flags (“cc”) as a precaution. > > #define alternative(oldinstr, newinstr, ft_flags) \ > asm_inline volatile (ALTERNATIVE(oldinstr, newinstr, ft_flags) : : : "memory") > > Actually there seems to be only one instance of problematic cases - in um/32-bit: > > #define mb() alternative("lock; addl $0,0(%%esp)", "mfence", X86_FEATURE_XMM2) > #define rmb() alternative("lock; addl $0,0(%%esp)", "lfence", X86_FEATURE_XMM2) > #define wmb() alternative("lock; addl $0,0(%%esp)", "sfence", X86_FEATURE_XMM) > > Presumably, if XMM or XMM2 are not supported, there would be instances where addl > would be able to change eflags arithmetic flags without the compiler being aware > of it. > > As it only affects 32-bit Linux UM - I don’t easily have an environment to test > the fix. An alternative (word-pun unintended) is to add “cc” as a precaution > to the alternative macro. > > > _______________________________________________ > linux-um mailing list > linux-um@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-um
Application alternatives in um is presently a NOP. It always uses the "blunt and heavy instrument" - the most conservative option.
It is on the TODO list.
-- Anton R. Ivanov https://www.kot-begemot.co.uk/
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