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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Wed, 10 Jun 2020 12:10:21 +0200
Message-ID: <CACRpkdboPBGBdHaRtZ=OAFvikYZVh=BVu192uyiB8T=yeHHEbA@mail.gmail.com>
Subject: Re: ARM: vmsplit 4g/4g
To:     afzal mohammed <afzal.mohd.ma@gmail.com>
Cc:     Russell King - ARM Linux admin <linux@armlinux.org.uk>,
        Arnd Bergmann <arnd@arndb.de>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Mon, Jun 8, 2020 at 1:09 PM afzal mohammed <afzal.mohd.ma@gmail.com> wrote:
> On Sun, Jun 07, 2020 at 05:11:16PM +0100, Russell King - ARM Linux admin wrote:
> > On Sun, Jun 07, 2020 at 06:29:32PM +0530, afzal mohammed wrote:
>
> > > get_user_pages_fast() followed by kmap_atomic() & then memcpy() seems
> > > to work in principle for user copy.
> >
> > Have you done any performance evaluation of the changes yet? I think
> > it would be a good idea to keep that in the picture. If there's any
> > significant regression, then that will need addressing.
>
> Not yet. Yes, i will do the performance evaluation.
>
> i am also worried about the impact on performance as these
> [ get_user_pages() or friends, kmap_atomic() ] are additionally
> invoked in the copy_{from,to}_user() path now.

I am happy to help!

I am anyway working on MMU-related code (KASan) so I need to be on
top of this stuff.

What test is appropriate for this? I would intuitively think hackbench?

> Note that this was done on a topic branch for user copy. Changes for
> kernel static mapping to vmalloc has not been merged with these.
> Also having kernel lowmem w/ a separate asid & switching at kernel
> entry/exit b/n user & kernel lowmem by changing ttbr0 is yet to be
> done. Quite a few things remaining to be done to achieve vmsplit 4g/4g

I will be very excited to look at patches or a git branch once you have
something you want to show. Also to just understand how you go about
this. I have several elder systems under my
roof so my contribution could hopefully be to help and debug any issues
on these.

Yours,
Linus Walleij