linux-kernel - ARM: vmsplit 4g/4g

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Date:   Sun, 7 Jun 2020 18:29:32 +0530
From:   afzal mohammed <afzal.mohd.ma@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Russell King <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: ARM: vmsplit 4g/4g

Hi,

On Sat, May 16, 2020 at 09:35:57AM +0200, Arnd Bergmann wrote:
> On Sat, May 16, 2020 at 8:06 AM afzal mohammed <afzal.mohd.ma@...il.com> wrote:

> > Okay, so the conclusion i take is,
> > 1. VMSPLIT 4G/4G have to live alongside highmem
> > 2. For user space copy, do pinning followed by kmap

> Right, though kmap_atomic() should be sufficient here
> because it is always a short-lived mapping.

get_user_pages_fast() followed by kmap_atomic() & then memcpy() seems
to work in principle for user copy.

Verified in a crude way by pointing TTBR0 to a location that has user
pgd's cleared upon entry to copy_to_user() & restoring TTBR0 to
earlier value after user copying was done and ensuring boot.

Meanwhile more testing w/ kernel static mapping in vmalloc space
revealed a major issue, w/ LPAE it was not booting. There were issues
related to pmd handling, w/ !LPAE those issues were not present as pmd
is in effect equivalent to pgd. The issues has been fixed, though now
LPAE boots, but feel a kind of fragile, will probably have to revisit
it.

Regards
afzal