Re: [Xen-devel] Re: [GIT PULL] xen /proc/mtrr implementation

[Gerd Hoffmann]

On 05/19/09 15:31, Ingo Molnar wrote:
> * Gerd Hoffmann<kraxel@xxxxxxxxxx>  wrote:
>
> > On 05/19/09 14:26, Ingo Molnar wrote:
> > > * Gerd Hoffmann<kraxel@xxxxxxxxxx>   wrote:
> > >
> > > > On 05/19/09 13:08, Ingo Molnar wrote:
> > > > > Or, alternatively, the hypervisor can expose its own administrative
> > > > > interface to manage MTRRs.
> > > > Guess what?  Xen does exactly that.  And the xen mtrr_ops
> > > > implementation uses that interface ...
> > > No, that is not an 'administrative interface' - that is a guest
> > > kernel level hack that complicates Linux, extends its effective ABI
> > > dependencies and which has to be maintained there from that point
> > > on.
> > >
> > > There's really just three proper technical solutions here:
> > >
> > > - either catch the lowlevel CPU hw ops (the MSR modifications, which
> > >     isnt really all that different from the mtrr_ops approach so it
> > >     shouldnt pose undue difficulties to the Xen hypervisor).
> > Devil is in the details.
> >
> > The dom0 kernel might not see all physical cpus on the system.  So
> > Xen can't leave the job of looping over all cpus to the dom0
> > kernel, Xen has to apply the changes made by the (priviledged)
> > guest kernel on any (virtual) cpu to all (physical) cpus in the
> > machine.
>
> Applying MTRR changes to only part of the CPUs is utter madness.

Sure.  Do you read what I'm writing?

> > Which in turn means the "lowlevel cpu hw op" would work in a
> > slightly different way on Xen and native.  Nasty.
> >
> > >     That will
> > >     be maximally transparent and compatible, with zero changes needed
> > >     to the Linux kernel.
> > No, the linux kernel probably should do the wrmsr on one cpu only then.
>
> Why?

See above.  Xen has to apply the changes to all cpus anyway.

> > Oops, the third "proper technical solutions" is missing.
>
> Yeah, the third one is to not touch MTRRs after bootup and use PAT.

Works only in case the CPU has PAT support.

cheers,
  Gerd

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