Re: [Xen-devel] XSA 154 and ISA region (640K -> 1MB) WB cache instead of UC

From: Jan Beulich
Date: Thu Aug 18 2016 - 07:12:58 EST


>>> On 18.08.16 at 12:16, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 18/08/16 11:06, Jan Beulich wrote:
>>>>> On 17.08.16 at 22:32, <konrad.wilk@xxxxxxxxxx> 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?

Jan