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On 13.06.15 at 01:15, [off-list ref] wrote:

On Jun 12, 2015 12:59 AM, "Jan Beulich" [off-list ref] wrote:

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On 12.06.15 at 01:23, [off-list 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.

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