Re: Access to non-RAM pages

[Linus Torvalds]

[ Adding a few new people the the cc.

  The issue is the worry about software-speculative accesses (ie
things like CONFIG_DCACHE_WORD_ACCESS - not talking about the hw
speculation now) accessing past RAM into possibly contiguous IO ]

On Sat, Sep 1, 2018 at 10:27 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> If you have a machine with RAM that touches IO, you need to disable
> the last page, exactly the same way we disable and marked reserved the
> first page at zero.
>
> I thought we already did that.

We don't seem to do that.

And it's not just the last page, it's _any_ last page in a region that
bumps up to IO. That's actually much more common in the low 4G area on
PC's, I suspect, although the reserved BIOS ranges always tend to be
there.

I suspect it should be trivial to do - maybe in
e820__memblock_setup()? That's where we already trim partial pages
etc.

In fact, I think this might be done as an extension of commit
124049decbb1 ("x86/e820: put !E820_TYPE_RAM regions into
memblock.reserved"), except making sure that non-RAM regions mark one
page _previous_ as reserved too.

I assume memory hotplug might have the same issue, and checking
whether ARM64 and powerpc perhaps might have already done something
like this (or might need to add it).

We discussed long ago the case of user space mapping IO in user space,
and decided we didn't care. But the kernel should probably explicitly
make sure we don't either, even if I can't recall having ever seen a
machine that actually maps IO contiguously to RAM. The layout always
tends to end up having holes anyway.

              Linus