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

- From: Ingo Molnar \<mingo@kernel.org\>
- Date: 2015-06-13 06:37:46 
- List: linux-fbdev
- Page: https://ratatoskr.run/linux-fbdev/2015/06/4515244
- Thread: https://ratatoskr.run/linux-fbdev/2015/06/4514267/t.md

```text

* Andy Lutomirski [ref] wrote:

> On Jun 12, 2015 12:59 AM, "Jan Beulich" [ref] wrote:
> >
> > >>> On 12.06.15 at 01:23, [ref] 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.

Not to mention suspend/resume, reboot and other goodies where the firmware might 
pop up expecting intact MTRRs.

Btw., doesn't a lack of MTRRs imply UC? So is 'crash and burn' possible in most 
cases? Isn't it just 'executes slower than before'?

Thanks,

	Ingo
```