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

Zhenyu Ye
2020-04-21T12:22:48Z

On 2020/4/21 16:30, Peter Zijlstra wrote:
> 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.

Thanks for your review. In my view, the asm-code should behave the same
as the C code, even if cleared_* are bitfields (below 02 optimization).

Below is the generated asm of my code (gcc version is 7.3.0):

<tlb_flush_mmu_tlbonly.part.110>:
...
ubfx x5, x2, #3, #1 // x2 stores the values of cleared_*
ubfx x1, x2, #4, #1
add w1, w1, w5
ubfx x5, x2, #5, #1
add w1, w1, w5
ubfx x2, x2, #6, #1
add w1, w1, w2 // then the w1 = sum of cleared_*
tbnz w3, #3, 001030f8b8
tbz w3, #4, 001030fac0
cmp w1, #0x1 // cmp the w1 to select branch
mov w5, #0x2
... // do the if-else below...

Then with your code above, the generated asm is:

<tlb_flush_mmu_tlbonly.part.110>:
...
tbnz w1, #3, 001030f8a0 // w1 stores the values of cleared_*
tbz w1, #4, 001030fac0
and w2, w1, #0x78 // mask the cleared_* to w2
mov x4, #0x200000
mov w7, #0x15
mov w6, #0x3
cmp w2, #0x8 // cmp the w2 to 0x8, 0x10, 0x20 to
// select branch
b.ne ffff80001030f8b8
... // do the if-else below...

So at the gen-asm level, both of our codes are OK. But your code is really
more saner than mine at the gen-asm level.

Thanks for your suggestion of this, I will send a new patch series soon.

Zhenyu

.