Date: Wed, 13 Apr 2016 23:03:10 +0200
From: "Luis R. Rodriguez" <mcgrof@kernel.org>
To: "Maciej W. Rozycki" <macro@linux-mips.org>
Cc: Ingo Molnar <mingo@kernel.org>, Toshi Kani <toshi.kani@hpe.com>,
        "Luis R. Rodriguez" <mcgrof@kernel.org>,
        Toshi Kani <toshi.kani@hp.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Dave Airlie <airlied@redhat.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        linux-arch@vger.kernel.org, X86 ML <x86@kernel.org>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>, Brian Gerst <brgerst@gmail.com>
Subject: Re: Overlapping ioremap() calls, set_memory_*() semantics
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On Mon, Mar 21, 2016 at 05:38:40PM +0000, Maciej W. Rozycki wrote:
> On Wed, 9 Mar 2016, Ingo Molnar wrote:
> 
> > > > So to go back to the original suggestion from Luis, I've quoted it, but
> > > > with a s/overlapping/aliased substitution:
> > > > 
> > > > > I had suggested long ago then that one possible resolution was for us
> > > > > to add an API that *enables* aliased ioremap() calls, and only use it
> > > > > on select locations in the kernel. This means we only have to convert a
> > > > > few users to that call to white list such semantics, and by default
> > > > > we'd disable aliased calls. To kick things off -- is this strategy
> > > > > agreeable for all other architectures?
> > > > 
> > > > I'd say that since the overwhelming majority of ioremap() calls are not
> > > > aliased, ever, thus making it 'harder' to accidentally alias is probably
> > > > a good idea.
> > > 
> > > Did you mean 'aliased' or 'aliased with different cache attribute'? ?The former 
> > > check might be too strict.
> > 
> > I'd say even 'same attribute' aliasing is probably relatively rare.
> 
>  Please note that aliased cached mappings (any kinds of, not necessarily 
> from `ioremap') cause a lot of headache (read: handling trouble) with 
> architectures such as MIPS which support virtually indexed caches which 
> suffer from cache aliasing.  There is a risk of data corruption if the 
> same physical memory address space location is accessed through different 
> virtual mappings as not all hardware catches duplicate cache entries 
> created in such a case.
> 
>  We handle it in software for user mappings (although I keep having a 
> feeling something always keeps escaping, due to the vast diversity of 
> cache configurations possible), however I don't think we do for `ioremap', 
> so disallowing aliased `ioremap' mappings by default sounds like a good 
> idea to me.

Great, well lets do the work then.

  Luis