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SubjectRe: Overlapping ioremap() calls, set_memory_*() semantics
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

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