linux-kernel - Re: xtensa question, was Re: [PATCH V1 00/10] Remove duplicated kmap code

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Message-ID: <20200501091933.GA3084@infradead.org>
Date:   Fri, 1 May 2020 02:19:33 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Max Filippov <jcmvbkbc@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>, ira.weiny@...el.com,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:TENSILICA XTENSA PORT (xtensa)" 
        <linux-xtensa@...ux-xtensa.org>
Subject: Re: xtensa question, was Re: [PATCH V1 00/10] Remove duplicated kmap
 code

On Fri, May 01, 2020 at 02:02:19AM -0700, Max Filippov wrote:
> Hi Christoph,
> 
> On Fri, May 1, 2020 at 1:46 AM Christoph Hellwig <hch@...radead.org> wrote:
> > any idea why xtensa uses PAGE_KERNEL_EXEC instead of PAGE_KERNEL
> > for kmap_prot?  Mapping all mapped highmem as executable seems rather
> > dangerous.
> 
> I sure do: to allow instruction cache flushing when writing to high user
> pages temporarily mapped with kmap. Instruction cache management
> opcodes that operate on virtual addresses would raise an exception if
> the address is not executable.

Seems like this should use kmap_atomic_prot with PAGE_KERNEL_EXEC just
for that case.  Which of course didn't exist on xtensa so far, but with
this series will.