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SubjectRe: Access to non-RAM pages
On Sun, 2 Sep 2018, Linus Torvalds wrote:

> Which basically doesn't happen on x86 in reality. BIOSes just don't put
> MMIO right after the last page of RAM. I think this is why it only
> triggered on Xen, due to some crazy "Xen reacts badly" case where we do
> the speculation into a balloon address.
>
> So _practically_ this is just a Xen bug, nothing more.

Yeah, and Xen guys are already working on fixing that AFAIK.

> But since in _theory_ you could have MMIO abut regular RAM directly,
> it's worth maybe making sure it's purely theory.

Well, we've been hit by similar/related issue in practice, on x86 machines
where GART aperture is being mapped over physical RAM. For the curious --
see commit 2a3e83c6f ("x86/gart: Exclude GART aperture from vmcore").

Thanks,

--
Jiri Kosina
SUSE Labs

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