[Xen-devel] Re: The memory type of non-RAM address is WB by default?

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 23/10/08 11:01, "Cui, Dexuan" <dexuan.cui@xxxxxxxxx> wrote:

> Keir Fraser wrote:
>> Why take any notice of guest MTRR/PAT in cases where the host knows
>> better?
>> 
> So we don't treat this warning as a bug and would not fix it. Correct?
> I don't like warning. :-)

Well, one approach would be not to warn! :-)

Seriously: for anything which is not guest RAM, the host should really know
best what memory type it should be mapped as (probably UC or WC). Hence why
mess around validating a guest-requested memory type? The situation is
different of course for guest RAM being used in non-WB contexts for device
communications: in that case the guest does know best.

The alternative is to give guests a shedload more MTRR ranges. This is kind
of clunky and will also slow down the MTRR-range search which Xen currently
does when working out guest effective memory type. But it does have the
advantage of letting us have default type as UC, which I guess is the
default type on just about every real machine in the world?

 -- Keir



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