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

On Thu, Apr 2, 2015 at 3:20 PM, Luis R. Rodriguez [off-list ref] wrote:

On Thu, Apr 2, 2015 at 1:13 PM, Bjorn Helgaas [off-list ref] wrote:

quoted

On Thu, Mar 26, 2015 at 6:35 PM, Luis R. Rodriguez [off-list ref] wrote:

quoted

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"?  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.

quoted

 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).

Bjorn