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

From: Luis R. Rodriguez
Date: Thu Apr 02 2015 - 16:21:22 EST

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.

> 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.

> Please use the conventional citation format:
> 47591df50512 ("xen: Support Xen pv-domains using PAT")
> A one-character typo in a SHA1 makes it completely useless, so it's
> nice to have the summary line both for readability and a bit of
> redundancy.

Sure, fixed.

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