linux-kernel - Re: [PATCH linux-next] KVM: selftests: remove redundant variable tsc

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Date:   Tue, 20 Sep 2022 19:30:44 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     cgel.zte@...il.com, pbonzini@...hat.com, shuah@...nel.org,
        dmatlack@...gle.com, peterx@...hat.com, oupton@...gle.com,
        kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Jinpeng Cui <cui.jinpeng2@....com.cn>,
        Zeal Robot <zealci@....com.cn>
Subject: Re: [PATCH linux-next] KVM: selftests: remove redundant variable
 tsc_val

On Wed, Aug 31, 2022, Jim Mattson wrote:
> On Wed, Aug 31, 2022 at 7:31 AM <cgel.zte@...il.com> wrote:
> > diff --git a/tools/testing/selftests/kvm/include/x86_64/processor.h b/tools/testing/selftests/kvm/include/x86_64/processor.h
> > index 0cbc71b7af50..75920678f34d 100644
> > --- a/tools/testing/selftests/kvm/include/x86_64/processor.h
> > +++ b/tools/testing/selftests/kvm/include/x86_64/processor.h
> > @@ -237,7 +237,6 @@ static inline uint64_t get_desc64_base(const struct desc64 *desc)
> >  static inline uint64_t rdtsc(void)
> >  {
> >         uint32_t eax, edx;
> > -       uint64_t tsc_val;
> >         /*
> >          * The lfence is to wait (on Intel CPUs) until all previous
> >          * instructions have been executed. If software requires RDTSC to be
> > @@ -245,8 +244,8 @@ static inline uint64_t rdtsc(void)
> >          * execute LFENCE immediately after RDTSC
> >          */
> >         __asm__ __volatile__("lfence; rdtsc; lfence" : "=a"(eax), "=d"(edx));
> > -       tsc_val = ((uint64_t)edx) << 32 | eax;
> > -       return tsc_val;
> > +
> > +       return ((uint64_t)edx) << 32 | eax;
> >  }
> 
> This does beg the question: "Why?"

Yeah, for this one I think having a local variable adds value.