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Date: Thu, 18 Aug 2016 16:35:44 +0100
From: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
To: "Jan Beulich" <JBeulich@suse.com>
Cc: "Andrew Cooper" <andrew.cooper3@citrix.com>,
        "Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
        <stefan.bader@canonical.com>, <david.vrabel@citrix.com>,
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        "Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
        <chuck.anderson@oracle.com>, "Juergen Gross" <JGross@suse.com>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [Xen-devel] XSA 154 and ISA region (640K -> 1MB) WB cache
 instead of UC
Message-ID: <20160818163544.07c8a051@lxorguk.ukuu.org.uk>
In-Reply-To: <57B5B4560200007800107092@prv-mh.provo.novell.com>
References: <20160817203238.GA9408@char.us.oracle.com>
        <57B5A4C90200007800106FF2@prv-mh.provo.novell.com>
        <47fabc70-4500-1035-7dc7-7f0a3915471f@citrix.com>
        <57B5B4560200007800107092@prv-mh.provo.novell.com>
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On Thu, 18 Aug 2016 05:12:54 -0600
"Jan Beulich" <JBeulich@suse.com> wrote:

> >>> On 18.08.16 at 12:16, <andrew.cooper3@citrix.com> wrote:  
> > On 18/08/16 11:06, Jan Beulich wrote:  
> >>>>> On 17.08.16 at 22:32, <konrad.wilk@oracle.com> wrote:  
> >>>    Looking at the kernel it assumes that WB is ok for 640KB->1MB.
> >>>    The comment says:
> >>>    " /* Low ISA region is always mapped WB in page table. No need to track   
> > *"  
> >> As per above it's not clear to me what this comment is backed by.  
> > 
> > This states what is in the pagetables.  Not the combined result with MTRRs.
> > 
> > WB in the pagetables and WC/UB in the MTRRs is a legal combination which
> > functions correctly.  
> 
> True, but then again - haven't I been told multiple times that Linux
> nowadays prefers to run without using MTRRs?

The BIOS sets up the fixed MTRR registers for the 640K-1MB window. Those
are separate to the variable range MTRR registers used for main memory
with specific mappings for segments A000 to BFFF then C000-C7FF /
C800-CFFF / etc up to FFFF.

Alan