Re: [PATCH 5/5] docs/pvh: document initial MTRR state

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, May 14, 2018 at 05:03:52PM +0100, Wei Liu wrote:
> On Thu, May 10, 2018 at 06:15:05PM +0100, Roger Pau Monne wrote:
> > Provided to both Dom0 and DomUs.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> > ---
> > Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> > Cc: George Dunlap <George.Dunlap@eu.citrix.com>
> > Cc: Ian Jackson <ian.jackson@eu.citrix.com>
> > Cc: Jan Beulich <jbeulich@suse.com>
> > Cc: Julien Grall <julien.grall@arm.com>
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> > Cc: Stefano Stabellini <sstabellini@kernel.org>
> > Cc: Tim Deegan <tim@xen.org>
> > Cc: Wei Liu <wei.liu2@citrix.com>
> > ---
> >  docs/misc/pvh.markdown | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> > 
> > diff --git a/docs/misc/pvh.markdown b/docs/misc/pvh.markdown
> > index e85fb15374..639401a887 100644
> > --- a/docs/misc/pvh.markdown
> > +++ b/docs/misc/pvh.markdown
> > @@ -92,3 +92,18 @@ event channels. Delivery of those interrupts can be configured in the same way
> >  as HVM guests, check xen/include/public/hvm/params.h and
> >  xen/include/public/hvm/hvm\_op.h for more information about available delivery
> >  methods.
> > +
> > +## MTRR ##
> > +
> > +### Unprivileged guests ###
> > +
> > +PVH guests are booted with the default MTRR type set to write-back and MTRR
> > +enabled. This allows DomUs to start with a sane MTRR state. Note that this will
> > +have to be revisited when pci-passthrough is added to PVH in order to set MMIO
> > +regions as UC.
> 
> My reading is "revisited" implies the default type will change. In fact
> it shouldn't. We should clarify: for ram it will remain WB, for MMIO
> holes it will be UC.
>
> Please correct me if I'm wrong.

That's correct. I've used "revisited" here in the sense that Xen might
change the default type to UC and set the RAM regions as WB using
variable MTRR ranges for example.

I simply wanted to remark that the way RAM is set to WB is currently
done using the default MTRR type. RAM will always be set of WB for PVH
in MTRR, however the way to achieve it might change.

What about adding:

"Xen guarantees that RAM regions will always have the WB cache type
set in the initial MTRR state, either set by the default MTRR type or
by other means."

Thanks, Roger.

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