linux-kernel - Re: [PATCH v2 5/6] mm: tlb: Provide flush_*_tlb

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Date:   Sat, 30 May 2020 18:24:21 +0800
From:   Zhenyu Ye <yezhenyu2@...wei.com>
To:     Catalin Marinas <catalin.marinas@....com>
CC:     <peterz@...radead.org>, <mark.rutland@....com>, <will@...nel.org>,
        <aneesh.kumar@...ux.ibm.com>, <akpm@...ux-foundation.org>,
        <npiggin@...il.com>, <arnd@...db.de>, <rostedt@...dmis.org>,
        <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 v2 5/6] mm: tlb: Provide flush_*_tlb_range wrappers

Hi Catalin,

Sorry for taking so long to reply to you.

On 2020/5/26 22:52, Catalin Marinas wrote:
> On Mon, May 25, 2020 at 03:19:42PM +0800, Zhenyu Ye wrote:
>>
>> tlb_flush_##_pxx##_range() is used to set tlb->cleared_*,
>> flush_##_pxx##_tlb_range() will actually flush the TLB entry.
>>
>> In arch64, tlb_flush_p?d_range() is defined as:
>>
>> 	#define flush_pmd_tlb_range(vma, addr, end)	flush_tlb_range(vma, addr, end)
>> 	#define flush_pud_tlb_range(vma, addr, end)	flush_tlb_range(vma, addr, end)
> 
> Currently, flush_p??_tlb_range() are generic and defined as above. I
> think in the generic code they can remain an alias for
> flush_tlb_range().
> 
> On arm64, we can redefine them as:
> 
> #define flush_pte_tlb_range(vma, addr, end)	__flush_tlb_range(vma, addr, end, 3)
> #define flush_pmd_tlb_range(vma, addr, end)	__flush_tlb_range(vma, addr, end, 2)
> #define flush_pud_tlb_range(vma, addr, end)	__flush_tlb_range(vma, addr, end, 1)
> #define flush_p4d_tlb_range(vma, addr, end)	__flush_tlb_range(vma, addr, end, 0)
> 
> (unless the compiler optimises away all the mmu_gather stuff in your
> macro above but they don't look trivial to me)
> 

I changed generic code before considering that other structures may also
use this feature, such as Power9. And Peter may want to replace all
flush_tlb_range() by tlb_flush() in the future, see [1] for details.

If only enable this feature on aarch64, your codes are better.

[1] https://lore.kernel.org/linux-arm-kernel/20200402163849.GM20713@hirez.programming.kicks-ass.net/

> Also, I don't see the new flush_pte_* and flush_p4d_* macros used
> anywhere and I don't think they are needed. The pte equivalent is
> flush_tlb_page() (we need to make sure it's not used on a pmd in the
> hugetlb context).
> 

flush_tlb_page() is used to flush only one page.  If we add the flush_pte_tlb_range(),
then we can use it to flush a range of pages in the future.

But flush_pte_* and flush_p4d_* macros are really not used anywhere.
I will remove them in next version of series, and add them if someone needs.

>> So even if we know the level here, we can not pass the value to tlbi
>> instructions (flush_tlb_range() is a common kernel interface and retro-fit it
>> needs lots of changes), according to Peter's suggestion, I finally decide to
>> pass the value of TTL by the tlb_gather_* frame.[1]
> 
> My comment was about the generic implementation using mmu_gather as you
> are proposing. We don't need to change the flush_tlb_range() interface,
> nor do we need to rewrite flush_p??_tlb_range().
> 

Thanks,
Zhenyu