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

Peter Zijlstra
2020-04-21T08:31:32Z

On Mon, Apr 20, 2020 at 08:06:16PM -0400, Steven Rostedt wrote:
> Peter Zijlstra <peterz@infradead.org> wrote:
> > On Fri, Apr 03, 2020 at 05:00:48PM +0800, Zhenyu Ye wrote:

> > > +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.

Not really the point; he wants to guarantee he only returns >0 when
there's a single bit set. But the thing is, cleared_* is a bitfield, and
I'm afraid that the above will result in some terrible code-gen.

Maybe something like:

if (tlb->cleared_ptes && !(tlb->cleared_pmds ||
tlb->cleared_puds ||
tlb->cleared_p4ds))
return 3;

if (tlb->cleared_pmds && !(tlb->cleared_ptes ||
tlb->cleared_puds ||
tlb->cleared_p4ds))
return 2;

if (tlb->cleared_puds && !(tlb->cleared_ptes ||
tlb->cleared_pmds ||
tlb->cleared_p4ds))
return 1;

return 0;

Which I admit is far too much typing, but I suspect it generates far
saner code (just a few masks and branches).

But maybe the compiler surprises us, what do I konw.