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Date:   Tue, 14 Jul 2020 09:12:11 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Christoph Hellwig <hch@lst.de>, Nick Hu <nickhu@andestech.com>,
        Greentime Hu <green.hu@gmail.com>,
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        Palmer Dabbelt <palmer@dabbelt.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [PATCH 5/6] uaccess: add force_uaccess_{begin,end} helpers
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On Mon, Jul 13, 2020 at 03:19:42PM +0200, Geert Uytterhoeven wrote:
> > This used to set KERNEL_DS, and now it sets USER_DS, which looks wrong
> > superficially.
> 
> Thanks for noticing, and sorry for missing that myself.
> 
> The same issue is present for SuperH:
> 
>     -               set_fs(KERNEL_DS);
>     +               oldfs = force_uaccess_begin();
> 
> So the patch description should be:
> 
>     "Add helpers to wraper the get_fs/set_fs magic for undoing any damage
>      done by set_fs(USER_DS)."
> 
> and leave alone users setting KERNEL_DS?

Yes, this was broken.  Fixed for the next version.

> > If the new behaviour is fine it suggests that the old behaviour was
> > wrong, or that this is superfluous and could go entirely.
> >
> > Geert?
> 
> Nope, on m68k, TLB cache operations operate on the current address space.
> Hence to flush a kernel TLB entry, you have to switch to KERNEL_DS first.
> 
> If we're guaranteed to be already using KERNEL_DS, I guess the
> address space handling can be removed.  But can we be sure?

We can't be sure yet.  But with a lot of my pending work we should be
able to get there in the not too far future.