linux-kernel - Re: [PATCH] x86/boot/64: Make level2_kernel_pgt pages invalid outside kernel area.

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Date:   Mon, 16 Sep 2019 12:00:58 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Steve Wahl <steve.wahl@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Juergen Gross <jgross@...e.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Jordan Borgner <mail@...dan-borgner.de>,
        Feng Tang <feng.tang@...el.com>, linux-kernel@...r.kernel.org,
        Baoquan He <bhe@...hat.com>, russ.anderson@....com,
        dimitri.sivanich@....com, mike.travis@....com
Subject: Re: [PATCH] x86/boot/64: Make level2_kernel_pgt pages invalid
 outside kernel area.

On Fri, Sep 13, 2019 at 10:14:15AM -0500, Steve Wahl wrote:
> On Thu, Sep 12, 2019 at 01:19:17PM +0300, Kirill A. Shutemov wrote:
> > On Wed, Sep 11, 2019 at 03:08:35PM -0500, Steve Wahl wrote:
> > > Thank you for your time looking into this with me!
> > 
> > With all this explanation the original patch looks sane to me.
> > 
> > But I would like to see more information from this thread in the commit
> > message and some comments in the code on why it's crucial not to map more
> > than needed.
> 
> I am working on this.
> 
> > I think we also need to make it clear that this is workaround for a broken
> > hardware: speculative execution must not trigger a halt.
> 
> I think the word broken is a bit loaded here.  According to the UEFI
> spec (version 2.8, page 167), "Regions that are backed by the physical
> hardware, but are not supposed to be accessed by the OS, must be
> returned as EfiReservedMemoryType."  Our interpretation is that
> includes speculative accesses.

+Dave.

I don't think it is. Speculative access is done by hardware, not OS.

BTW, isn't it a BIOS issue?

I believe it should have a way to hide a range of physical address space
from OS or force a caching mode on to exclude it from speculative
execution. Like setup MTRRs or something.

> I'd lean more toward "tightening of adherence to the spec required by
> some particular hardware."  Does that work for you?

Not really, no. I still believe it's issue of the platform, not OS.

-- 
 Kirill A. Shutemov