From: Nadav Amit
> Sent: 04 November 2023 09:41
>
> > On Nov 4, 2023, at 11:34 AM, Anton Ivanov <anton.iva...@kot-begemot.co.uk>
> > wrote:
> >
> > 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.
> >>
> > 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.
>
> Thanks for the quick response. But I don’t see how it prevents the problem
> (it actually makes it worse - affecting XMM/XMM2 CPUs as well) since you
> keep the “lock; addl $0,0(%%esp)” in the running code, affecting eflags
> without telling the compiler that you do so through a “cc” clobber.
gcc always assumes that inline asm changes "cc" - there is no need
to add a 'clobber' for it.
David
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