Re: pgd/pmd/pte and x86 kernel virtual addresses

["Benjamin C.R. LaHaise" <blah@kvack.org>]

On Fri, 25 Aug 2000, Timur Tabi wrote:

> ** Reply to message from "Benjamin C.R. LaHaise" <blah@kvack.org> on Fri, 25
> Aug 2000 12:45:18 -0400 (EDT)
> 
> 
> > > What I'm trying to do is allocate some memory via get_free_pages, and then mark
> > > that memory as uncacheable.
> > 
> > ioremap_nocache should be able to do what you want.
> 
> Well, that's what I tried to explain in my previous email which people seem to
> be ignoring.
> 
> I tried ioremap_nocache, and it doesn't appear to be working.  There are a
> number of problems:
> 
> 1) I'm trying to mark regular RAM as uncacheable, and ioremap_nocache()
> requires me to munge the PG_Reservered bit for each page before I can do that. 
> Ugly.

Yeap, that's right.

> 2) ioremap_nocache() allocates virtual RAM.  I already have a virtual address,
> I don't need another one.

That's because there are no ptes for most RAM in the kernel.
ioremap_nocache does not allocate RAM, only a mapping for the address
space.  Actually, passing in physical RAM to ioremap_nocache may not work
on all platforms.

> 3) The unmap function for ioremap_nocache() is a no-op.  So after I remap and
> mark the page as uncacheable, there's no way to restore it after I'm done!

Bummer.

> 4) Even with all this, it appears that the function isn't working.  I've
> attached a logical analyzer to the memory bus, and writes are not being sent
> out, leading me to believe the memory is still being cached.

On x86, you're better off setting the MTRRs to get non-cached behaviour,
but that's still the wrong thing to do when you're talking about main
memory.  Better still is to not rely on uncachable mappings at all.  x86
is a cache coherent architechure -- why do you need uncachable mappings of
main memory???. 

		-ben

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