Re: [PATCH v1 6/6] arm64: tlb: Set the TTL field in flush_tlb_range

Steven Rostedt
2020-04-21T00:06:22Z

On Mon, 20 Apr 2020 14:10:55 +0200
Peter Zijlstra <peterz@infradead.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@huawei.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