Re: [Xen-devel] [PATCH v1 02/47] x86: mtrr: generalize run time disabling of MTRR

["Luis R. Rodriguez" <mcgrof@xxxxxxxx>]

On Thu, Apr 02, 2015 at 03:28:51PM -0500, Bjorn Helgaas wrote:
> On Thu, Apr 2, 2015 at 3:20 PM, Luis R. Rodriguez <mcgrof@xxxxxxxx> wrote:
> > On Thu, Apr 2, 2015 at 1:13 PM, Bjorn Helgaas <bhelgaas@xxxxxxxxxx> wrote:
> >>
> >> On Thu, Mar 26, 2015 at 6:35 PM, Luis R. Rodriguez <mcgrof@xxxxxxxx> wrote:
> >>
> >> > I'll rephrase this to:
> >> >
> >> > ---
> >> > It is possible to enable CONFIG_MTRR and up with it
> >> > disabled at run time and yet CONFIG_X86_PAT continues
> >> > to kick through with all functionally enabled. This
> >> > can happen for instance on Xen where MTRR is not
> >> > supported but PAT is, this can happen now on Linux as
> >> > of commit 47591df50 by Juergen introduced as of v3.19.
> >>
> >> I still can't parse this.  What does "up with it disabled at run time"
> >> mean?
> >
> > It  means that technically even if your CPU/BIOS/system did support
> > MTRR if you use a kernel with MTRR support enabled you might end up
> > with a situation where under one situation MTRR  might be enabled and
> > at another run time scenario with the same exact kernel and system you
> > will end up with MTRR disabled. Such is the case for example when
> > booting with Xen, which disables the CPU bits on the hypervisor code.
> > If you boot the same system without Xen you'll get MTRR.
> 
> Your text is missing some words.  You seem to be using "up" as a verb,
> but it's not a verb.  Maybe you meant "end up"? 

Indeed.

> Even then, it
> wouldn't make sense for CONFIG_MTRR to be "disabled at run time"
> because CONFIG_MTRR is a compile-time switch.  The MTRR
> *functionality* could certainly be disabled at run-time, but not
> CONFIG_MTRR itself.

I'll clarify.

> >>  And "... continues to kick through"?  Probably some idiomatic
> >> usage I'm just too old to understand :)
> >
> > That means for example that in both the above circumstances even if
> > MTRR went disabled at run time with Xen, the kernel went through with
> > getting PAT enabled.
> 
> "CONFIG_X86_PAT continues to kick through" doesn't seem a very precise
> way of describing this.  But maybe it's enough for experts in this
> area (which I'm not).

I've rephrased this to:

---
It is possible to enable CONFIG_MTRR and CONFIG_X86_PAT                         
and end up with a system with MTRR functionality disabled                       
PAT functionality enabled. This can happen for instance                         
on Xen where MTRR is not supported but PAT is. This can                         
happen on Linux as of commit 47591df50 ("xen: Support Xen                       
pv-domains using PAT") by Juergen, introduced as of v3.19.
---

  Luis

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