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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan



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