Re: [Xen-devel] RIP MTRR - status update for upcoming v4.2

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Jun 12, 2015 12:59 AM, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>
> >>> On 12.06.15 at 01:23, <toshi.kani@xxxxxx> wrote:
> > There are two usages on MTRRs:
> >  1) MTRR entries set by firmware
> >  2) MTRR entries set by OS drivers
> >
> > We can obsolete 2), but we have no control over 1).  As UEFI firmwares
> > also set this up, this usage will continue to stay.  So, we should not
> > get rid of the MTRR code that looks up the MTRR entries, while we have
> > no need to modify them.
> >
> > Such MTRR entries provide safe guard to /dev/mem, which allows
> > privileged user to access a range that may require UC mapping while
> > the /dev/mem driver blindly maps it with WB.  MTRRs converts WB to UC in
> > such a case.
>
> But it wouldn't be impossible to simply read the MTRRs upon boot,
> store the information, disable MTRRs, and correctly use PAT to
> achieve the same effect (i.e. the "blindly maps" part of course
> would need fixing).

This may crash and burn badly when we call a UEFI function or an SMI
happens.  I think we should just leave the MTRRs alone.

--Andy

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