Re: [PATCH v3 1/4] Revert "Documentation/features: mark BATCHED_UNMAP_TLB_FLUSH doesn't apply to ARM64"

[Barry Song <21cnbao@gmail.com>]

On Fri, Sep 9, 2022 at 4:26 PM Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
>
>
>
> On 8/22/22 13:51, Yicong Yang wrote:
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > This reverts commit 6bfef171d0d74cb050112e0e49feb20bfddf7f42.
> >
> > I was wrong. Though ARM64 has hardware TLB flush, but it is not free
> > and it is still expensive.
> > We still have a good chance to enable batched and deferred TLB flush
> > on ARM64 for memory reclamation. A possible way is that we only queue
> > tlbi instructions in hardware's queue. When we have to broadcast TLB,
> > we broadcast it by dsb. We just need to get adapted the existing
> > BATCHED_UNMAP_TLB_FLUSH.
> >
> > Tested-by: Xin Hao <xhao@linux.alibaba.com>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> > ---
> >  Documentation/features/arch-support.txt        | 1 -
> >  Documentation/features/vm/TLB/arch-support.txt | 2 +-
> >  2 files changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/Documentation/features/arch-support.txt b/Documentation/features/arch-support.txt
> > index 118ae031840b..d22a1095e661 100644
> > --- a/Documentation/features/arch-support.txt
> > +++ b/Documentation/features/arch-support.txt
> > @@ -8,5 +8,4 @@ The meaning of entries in the tables is:
> >      | ok |  # feature supported by the architecture
> >      |TODO|  # feature not yet supported by the architecture
> >      | .. |  # feature cannot be supported by the hardware
> > -    | N/A|  # feature doesn't apply to the architecture
> >
> > diff --git a/Documentation/features/vm/TLB/arch-support.txt b/Documentation/features/vm/TLB/arch-support.txt
> > index 039e4e91ada3..1c009312b9c1 100644
> > --- a/Documentation/features/vm/TLB/arch-support.txt
> > +++ b/Documentation/features/vm/TLB/arch-support.txt
> > @@ -9,7 +9,7 @@
> >      |       alpha: | TODO |
> >      |         arc: | TODO |
> >      |         arm: | TODO |
> > -    |       arm64: | N/A  |
> > +    |       arm64: | TODO |
> >      |        csky: | TODO |
> >      |     hexagon: | TODO |
> >      |        ia64: | TODO |
>
> I believe this patch is not needed, which explicitly reverts an
> older commit. Instead when ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
> framework gets enabled on arm64, the same patch could just drop
> 'NA' as possible values for arch support for a give feature in
> file Documentation/features/arch-support.txt.

Sure. it is certainly ok to fix this in
arm64: support batched/deferred tlb shootdown during page reclamation

By a separate patch, I was trying to highlight that my previous patch was
wrong. but, yes. it is not fundamentally necessary.

Thanks
Barry