linux-kernel - Re: [PATCH 0/3] x86/mtrr, pat: make PAT independent from MTRR

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Date:   Fri, 9 Aug 2019 19:51:17 +0000
From:   "Kani, Toshi" <toshi.kani@....com>
To:     "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "bp@...en8.de" <bp@...en8.de>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "fei1.li@...el.com" <fei1.li@...el.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH 0/3] x86/mtrr, pat: make PAT independent from MTRR

On Fri, 2019-08-09 at 09:06 +0200, Borislav Petkov wrote:
> On Thu, Aug 08, 2019 at 08:54:17PM -0700, Isaku Yamahata wrote:
> > Make PAT(Page Attribute Table) independent from
> > MTRR(Memory Type Range Register).
> > Some environments (mainly virtual ones) support only PAT, but not MTRR
> > because PAT replaces MTRR.
> > It's tricky and no gain to support both MTRR and PAT except compatibility.
> > So some VM technologies don't support MTRR, but only PAT.

I do not think it is technically correct on bare metal.  AFAIK, MTRR is
still the only way to setup cache attribute in real-mode, which BIOS SMI
handler relies on in SMM.

> > This patch series makes PAT available on such environments without MTRR.
> 
> And this "justification" is not even trying. Which "VM technologies" are
> those? Why do we care? What's the impact? Why do we want this?
> 
> You need to sell this properly.

Agreed.  If the situation is still the same, Xen does not support MTRR,
and the kernel sets the PAT table to the BIOS hand-off state when MTRR
is disabled.  The change below accommodated the fact that Xen hypervisor
enables WC before hand-off, which is different from the default BIOS
hand-off state.  The kernel does not support setting PAT when MTRR is
disabled due to the dependency Isaku mentioned.

https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1107094.html

-Toshi