Re: [PATCH 2/2] x86/mtrr: Refactor PAT initialization code

From: Toshi Kani
Date: Tue Mar 29 2016 - 16:54:44 EST

On Tue, 2016-03-29 at 10:14 -0700, Luis R. Rodriguez wrote:
> On Fri, Mar 18, 2016 at 2:35 PM, Toshi Kani <toshi.kani@xxxxxxx> wrote:
> > On Thu, 2016-03-17 at 17:06 -0700, Luis R. Rodriguez wrote:
> > > On Mar 17, 2016 2:04 PM, "Toshi Kani" <toshi.kani@xxxxxxx> wrote:
> > > >
> >
> > I do not see any issue for Xen, but sure, we can discuss about Xen in a
> > separate thread.
> While on point -- I'll just wanted to clarify that a while ago you had
> hinted we needed to have Xen return a valid type with
> mtrr_type_lookup(), I then also explained how Xen disables MTRR [0],
> it was however unclear if you still believe mtrr_type_lookup() is
> needed on the guest side. Jan had pointed out that the Xen Hypervisor
> implements the XENPF_read_memtype hypercall. On the recent thread I
> posted [1] I got into my review of the prospects of implementing
> support for using this hypercall on Linux xen guests and issues and
> concerns with it. Please feel free to follow up there and we can take
> up the other items below here as they relate to bare metal.

What I said in the email [0] was that "When MTRRs are enabled, the kernel
needs to check through mtrr_type_lookup()".

Since Xen guests have MTRRs disabled, this statement does not apply. It
returns MTRR_TYPE_INVALID when disabled, and this is fine for the guests.

> [0]
> [1]
> iepoTNg@xxxxxxxxxxxxxx
> > > > > On x86 Linux code we now have ioremap_uc() that can't use MTRR
> > > > > behind the scenes, why would something like this on the BIOS not
> > > > > be possible? That ultimately uses set_pte_at(). What limitations
> > > > > are there on the BIOS that prevent us from just using strong UC
> > > > > for PAT on the BIOS?
> > > >
> > > > Because it requires to run in virtual mode with page tables.
> > >
> > > Ah... interesting... is UC really needed, what is the default? If the
> > > default is used would there be an issue ? Can such work be deferred
> > > to a later time ? It seems like a high burden to require on large
> > > piece of legacy architecture to just blow a fan.
> >
> > The default cache attribute (i.e. ranges not covered by MTRRs) is
> > specified by the MTRR default type MSR.
> Do we really need UC for the fan?

When you say "we", are you referring Xen guests? ÂXen guests do not need to
control the fan, so they do not need UC set in MTRRs.

In general, yes, MMIO registers need UC when they need to be accessed.

> What is the default for PAT?

There is no such thing as the default for PAT.Â

> Can't
> the same be used so that we way by default all ranges match what is
> also the default by PAT? Would that really break fan control ? If we
> have a match should't we be able to not have to worry about MTRRs at
> all in-kernel even on bare metal?

We do not need to know about BIOS impl, such as fan control, etc. ÂThe
point is that if BIOS sets MTRRs, then the kernel keeps their setup. ÂIf
(virtual) BIOS does not enable MTRRs, the kernel keeps them disabled. ÂWe
just need not to mess with the setup.

> Another option, which I've alluded to on the Xen thread is skipping
> over the MTRR space from the e820 map. Is that not possible ? This
> could be last resort... but which I'm hinting more for the Xen side of
> things if we *really* need get_mtrr() on the Xen guest side of
> things...

There is no MTRR space in the e820 map since they are MSRs. ÂSince Xen
guests disable MTRRs, I do not think you have any issue here...