Re: [PATCH RESEND 1/2] x86/locking: Use ALT_OUTPUT_SP() for percpu_{,try_}cmpxchg{64,128}_op()

[Uros Bizjak <ubizjak@gmail.com>]

On Thu, Feb 13, 2025 at 9:43 PM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 2/13/25 11:14, Uros Bizjak wrote:
> > percpu_{,try_}cmpxchg{64,128}() macros use CALL instruction inside
> > asm statement in one of their alternatives. Use ALT_OUTPUT_SP()
> > macro to add required dependence on %esp register.
>
> Is this just a pedantic fix? Or is there an actual impact to end users
> that needs to be considered?

When call insn is embedded in the asm, then the compiler doesn't know
that due to call insn asm now depends on stack pointer or frame
pointer, so it is free to schedule the instruction outside the
function frame prologue/epilogue. Currently, this only triggers
objtool warning, but if we ever compile the kernel with redzone (IIRC,
it was mentioned that this is possible with FRED enabled kernel), the
call will clobber the redzone. Please note that alternative_call()
family of functions, __alternative_atomic64() and
__arch_{,try_}cmpxchg64_emu() all use the same macro exactly for the
reason explained above.

OTOH, all recent x86_64 processors support CMPXCHG128 insn, so the
call alternative will be rarely used.

> Basically, you've told me what the patch does, but not why anyone should
> care or why it should be applied.

This is actually explained at length in the comment for
ASM_CALL_CONSTRAINT, which ALT_OUTPUT_SP macro uses.

Uros.