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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, May 14, 2018 at 10:13:47AM -0600, Jan Beulich wrote:
> >>> On 14.05.18 at 18:03, <wei.liu2@xxxxxxxxxx> wrote:
> > On Thu, May 10, 2018 at 06:15:05PM +0100, Roger Pau Monne wrote:
> >> --- 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.
> 
> Why would changing the default late be a problem? A firmware update on
> bare hardware might also have such an effect. The default type read from
> the MSR must not change across the lifetime of a VM, but imo may change
> across reboots of it.
> 

Then setting a default here doesn't really help OS developers because
they will always need to write code to set the correct type -- not that
this is a big issue, but as I understand it the point here is to avoid
that.

Wei.

> Jan
> 
> 

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