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

["Jan Beulich" <JBeulich@suse.com>]

>>> On 13.06.15 at 01:15, <luto@amacapital.net> wrote:
> On Jun 12, 2015 12:59 AM, "Jan Beulich" <JBeulich@suse.com> wrote:
>>
>> >>> On 12.06.15 at 01:23, <toshi.kani@hp.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