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Date:   Mon, 9 Oct 2023 09:29:50 -0700
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Subject: Re: [PATCH] perf/x86/p4: Fix "Wunused-but-set-variable" warning
From:   Sean Christopherson <seanjc@google.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Lucy Mielke <lucymielke@icloud.com>, mingo@redhat.com,
        acme@kernel.org, mark.rutland@arm.com,
        alexander.shishkin@linux.intel.com, jolsa@kernel.org,
        namhyung@kernel.org, irogers@google.com, adrian.hunter@intel.com,
        tglx@linutronix.de, bp@alien8.de, dave.hansen@linux.intel.com,
        x86@kernel.org, hpa@zytor.com, linux-perf-users@vger.kernel.org,
        linux-kernel@vger.kernel.org
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On Sat, Oct 07, 2023, Peter Zijlstra wrote:
> On Sat, Oct 07, 2023 at 08:55:19PM +0200, Lucy Mielke wrote:
> > This fixes a compiler warning when compiling an allyesconfig with W=3D1=
:
> > warning: variable =C2=B4high=C2=B4 set but not used [-Wunused-but-set-v=
ariable]
>=20
> What compiler and what .config?
>=20
> > Signed-off-by: Lucy Mielke <lucymielke@icloud.com>
> > ---
> >  arch/x86/events/intel/p4.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >=20
> > diff --git a/arch/x86/events/intel/p4.c b/arch/x86/events/intel/p4.c
> > index 35936188db01..69aaf7c0f340 100644
> > --- a/arch/x86/events/intel/p4.c
> > +++ b/arch/x86/events/intel/p4.c
> > @@ -1366,7 +1366,7 @@ static __initconst const struct x86_pmu p4_pmu =
=3D {
> > =20
> >  __init int p4_pmu_init(void)
> >  {
> > -	unsigned int low, high;
> > +	unsigned int low, __maybe_unused high;
> >  	int i, reg;
> > =20
> >  	/* If we get stripped -- indexing fails */
>=20
> Right after this we have:
>=20
> 	rdmsr(MSR_IA32_MISC_ENABLE, low, high);
>=20
> which should get high unconditionally used. If there's a problem then
> it's probably inside that rdmsr macro.

rdmsr() writes to "high", but nothing ever reads from high.  FWIW, I would =
_love_
for rdmsrl() to have return semantics, e.g. to be able to do:

	low  =3D (u32)rdmsrl(MSR_IA32_MISC_ENABLE);

or even

	if (!(rdmsrl(MSR_IA32_MISC_ENABLE) & BIT(7)))