From mboxrd@z Thu Jan  1 00:00:00 1970
From: Toshi Kani <toshi.kani@hpe.com>
Subject: Re: [PATCH 2/2] x86/mtrr: Refactor PAT initialization code
Date: Mon, 11 Apr 2016 08:30:13 -0600
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To: "Luis R. Rodriguez" <mcgrof@kernel.org>
Cc: Juergen Gross <jgross@suse.com>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, Matt Fleming <matt@codeblueprint.co.uk>, Olof Johansson <olof@lixom.net>, Paul Stewart <pstew@chromium.org>, Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Paul Gortmaker <paul.gortmaker@windriver.com>, Paul McKenney <paulmck@linux.vnet.ibm.com>, X86 ML <x86@kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, xen-devel@lists.xensource.com
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On Sat, 2016-04-09 at 04:04 +0200, Luis R. Rodriguez wrote:
> On Thu, Mar 17, 2016 at 03:56:47PM -0600, Toshi Kani wrote:
> >=20
> > On Wed, 2016-03-16 at 00:29 +0100, Luis R. Rodriguez wrote:
> > >=20
> > > On x86 Linux code we now have ioremap_uc() that can't use MTRR be=
hind
> > > the scenes, why would something like this on the BIOS not be
> > > possible? That ultimately uses set_pte_at(). What limitations are
> > > there on the BIOS that prevent us from just using strong UC for P=
AT
> > > on the BIOS?
>>
> > Because it requires to run in virtual mode with page tables.
>
> I see now. Specifically, BIOSes run in real mode, and PAT uses
> paging. Paging requires bit 31 on CR0 set (PG), and PG has no
> effect if the PE flag (Protection Enable) bit 0 on CR0 is clear.
> If PE is clear we have real mode, which is what the BIOS uses.
>=20
> Stupid question then:
>=20
> are there no use case for a BIOS to enter PE, even if just
> limited to set paging attributes for instance. For the simple
> sake of burying MTRR this seems worthy, but I wonder if there
> are other paging needs a BIOS might find use for.

The OS calls EFI in virtual mode. =C2=A0BIOS SMI handler, however, is c=
alled in
physical mode. =C2=A0Since it has the highest priority, it needs to fin=
ish as
soon as possible. =C2=A0So, it typically does not make additional steps=
 to
switch to virtual mode.

Thanks,
-Toshi