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

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Message-Id: <557E8ABA02000078000849A8@mail.emea.novell.com>
Date:	Mon, 15 Jun 2015 07:20:10 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Andy Lutomirski" <luto@...capital.net>
Cc:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	"Bjorn Helgaas" <bhelgaas@...gle.com>,
	"Jej B" <James.Bottomley@...senpartnership.com>,
	"Toshi Kani" <toshi.kani@...com>, "X86 ML" <x86@...nel.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	<ville.syrjala@...ux.intel.com>,
	"Julia Lawall" <julia.lawall@...6.fr>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"Dave Airlie" <airlied@...hat.com>, <syrjala@....fi>,
	"Juergen Gross" <JGross@...e.com>,
	"Luis Rodriguez" <Mcgrof@...e.com>, "Borislav Petkov" <bp@...e.de>,
	"Tomi Valkeinen" <tomi.valkeinen@...com>,
	"linux-fbdev" <linux-fbdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	<linux-media@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [Xen-devel] RIP MTRR - status update for upcoming v4.2

>>> On 13.06.15 at 01:15, <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.

I buy the SMI part, but UEFI runtime calls are being done on
page tables we construct and control, so attributes could be kept
correct without relying on MTRRs.

Jan

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