Re: Access to non-RAM pages

[Jiri Kosina]

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