linux-kernel - Re: [PATCH v1 6/6] arm64: tlb: Set the TTL field in flush_tlb

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Message-ID: <20200420200616.44c7c7ea@oasis.local.home>
Date:   Mon, 20 Apr 2020 20:06:16 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Zhenyu Ye <yezhenyu2@...wei.com>, mark.rutland@....com,
        will@...nel.org, catalin.marinas@....com,
        aneesh.kumar@...ux.ibm.com, akpm@...ux-foundation.org,
        npiggin@...il.com, arnd@...db.de, maz@...nel.org,
        suzuki.poulose@....com, tglx@...utronix.de, yuzhao@...gle.com,
        Dave.Martin@....com, steven.price@....com, broonie@...nel.org,
        guohanjun@...wei.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, arm@...nel.org, xiexiangyou@...wei.com,
        prime.zeng@...ilicon.com, zhangshaokun@...ilicon.com,
        kuhn.chenqun@...wei.com
Subject: Re: [PATCH v1 6/6] arm64: tlb: Set the TTL field in flush_tlb_range

On Mon, 20 Apr 2020 14:10:55 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, Apr 03, 2020 at 05:00:48PM +0800, Zhenyu Ye wrote:
> > This patch uses the cleared_* in struct mmu_gather to set the
> > TTL field in flush_tlb_range().
> > 
> > Signed-off-by: Zhenyu Ye <yezhenyu2@...wei.com>
> > ---
> >  arch/arm64/include/asm/tlb.h      | 26 +++++++++++++++++++++++++-
> >  arch/arm64/include/asm/tlbflush.h | 14 ++++++++------
> >  2 files changed, 33 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/tlb.h b/arch/arm64/include/asm/tlb.h
> > index b76df828e6b7..d5ab72eccff4 100644
> > --- a/arch/arm64/include/asm/tlb.h
> > +++ b/arch/arm64/include/asm/tlb.h
> > @@ -21,11 +21,34 @@ static void tlb_flush(struct mmu_gather *tlb);
> >  
> >  #include <asm-generic/tlb.h>
> >  
> > +/*
> > + * get the tlbi levels in arm64.  Default value is 0 if more than one
> > + * of cleared_* is set or neither is set.
> > + * Arm64 doesn't support p4ds now.
> > + */
> > +static inline int tlb_get_level(struct mmu_gather *tlb)
> > +{
> > +	int sum = tlb->cleared_ptes + tlb->cleared_pmds +
> > +		  tlb->cleared_puds + tlb->cleared_p4ds;
> > +
> > +	if (sum != 1)
> > +		return 0;
> > +	else if (tlb->cleared_ptes)
> > +		return 3;
> > +	else if (tlb->cleared_pmds)
> > +		return 2;
> > +	else if (tlb->cleared_puds)
> > +		return 1;
> > +
> > +	return 0;
> > +}  
> 
> That's some mighty wonky code. Please look at the generated asm.

Without even looking at the generated asm, if a condition returns,
there's no reason to add an else for that condition.

	if (x)
		return 1;
	else if (y)
		return 2;
	else
		return 3;

Is exactly the same as:

	if (x)
		return 1;
	if (y)
		return 2;
	return 3;

-- Steve