linux-kernel - Re: Access to non-RAM pages

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Message-ID: <nycvar.YFH.7.76.1809030801550.25787@cbobk.fhfr.pm>
Date:   Mon, 3 Sep 2018 08:05:12 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Jürgen Groß <jgross@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Will Deacon <will.deacon@....com>
Subject: Re: 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