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

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Date:	Sat, 13 Jun 2015 08:37:32 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Jan Beulich <JBeulich@...e.com>, Juergen Gross <jgross@...e.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	Ville Syrjälä <syrjala@....fi>,
	Dave Airlie <airlied@...hat.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	linux-fbdev <linux-fbdev@...r.kernel.org>,
	X86 ML <x86@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jej B <James.Bottomley@...senpartnership.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	linux-media@...r.kernel.org, Luis Rodriguez <Mcgrof@...e.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Toshi Kani <toshi.kani@...com>, Borislav Petkov <bp@...e.de>,
	Julia Lawall <julia.lawall@...6.fr>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	ville.syrjala@...ux.intel.com
Subject: Re: [Xen-devel] RIP MTRR - status update for upcoming v4.2


* Andy Lutomirski <luto@...capital.net> wrote:

> On Jun 12, 2015 12:59 AM, "Jan Beulich" <JBeulich@...e.com> wrote:
> >
> > >>> On 12.06.15 at 01:23, <toshi.kani@...com> 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
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