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

[Toshi Kani]

On Fri, 2015-06-12 at 08:59 +0100, Jan Beulich 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).

It could be done, but I do not see much benefit of doing it.  One of the
reasons platform vendors set MTRRs is so that a system won't hit a
machine check when an OS bug leads an access with a wrong cache type.  A
machine check is hard to analyze and can be seen as a hardware issue by
customers.  Emulating MTRRs with PAT won't protect from such a bug. 

> > UEFI memory table has memory attribute, which describes cache types
> > supported in physical memory ranges.  However, this information gets
> > lost when it it is converted to e820 table.
> 
> I'm afraid you rather don't want to trust that information, as
> firmware vendors frequently screw it up.

Could be, but we need to use firmware info when necessary...

Thanks,
-Toshi

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