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

[Toshi Kani <toshi.kani@xxxxxx>]

On Fri, 2015-06-12 at 02:52 +0200, Luis R. Rodriguez wrote:
> On Thu, Jun 11, 2015 at 05:23:16PM -0600, Toshi Kani wrote:
> > On Thu, 2015-06-11 at 13:36 -0700, Luis R. Rodriguez wrote:
> >  :
> > > Pending RIP MTRR patches
> > > ====================
> > > 
> > > There are a few pending series so I wanted to provide a status update
> > > on those series.
> > > 
> > > mtrr: bury MTRR - unexport mtrr_add() and mtrr_del()
> > > 
> > > This is the nail on the MTRR coffin, it will prevent future direct
> > > access to MTRR code. This will not be posted until all of the below
> > > patches are in and merged. A possible next step here might be to
> > > consider separating PAT code from MTRR code and making PAT a first
> > > class citizen, enabling distributions to disable MTRR code in the
> > > future. I thought this was possible but for some reason I recently
> > > thought that there was one possible issue to make this happen. I
> > > suppose we won't know unless we try, unless of course someone already
> > > knows, Toshi?
> > 
> > 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.
> > 
> > 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.
> 
> Is there no way to modify CPU capability bits upon boot and kick UEFI
> to re-evaluate ? In such UEFI cases what happens for instance when
> Xen is used which does not support MTRR?

EFI GetMemoryMap() is a boot service, and won't be available after
ExitBootServices() is called.  But we should be able to keep the
attribute information copied into some table if necessary.

Xen provides virtual firmware on their guests, right?  If this firmware
does not set up MTRRs today, then I do not think it needs to set up for
UEFI, either.  Assuming the guest physical address is virtualized, it
does not have to carry the same platform attribute & restriction.

Thanks,
-Toshi



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