RE: [Xen-ia64-devel] TR virtualization vs. DCR

["Dong, Eddie" <eddie.dong@xxxxxxxxx>]

Dan and Ttristan:
        Yes, assuming guest OS will not have such kind of corner behavior is 
one possibility for paravirtualization, but at least setting of guest DCR 
should go to machine DCR otherwise either result in bad performance (guest want 
deferral but machine not) or correctness issue (guest want no deferral but 
machine defers). Also disabling machine spotaneous deferral is a good way to 
avoid a lot of corner cases.
        We are revising the design holes before starting Windows up.  If we 
make sure the VMM is architecturally complete, then windows up should happen 
automatically.  That will be cool. 
        Yes, sometime we are taking shortcut, but we need to be aware of the 
design holes at least among those key developers or document them.
Thx,eddie

Magenheimer, Dan (HP Labs Fort Collins) wrote:
>> Le Mercredi 12 Octobre 2005 10:35, Dong, Eddie a écrit :
>>> Dan and All:
>>> 
>>>      Currently both VTI and non-VTI are using machine TC to emulate
>>> guest TR, that is a must as machine memory a guest get may be
>>> discontiguous and may not be proper aligned. But this introduces
>>> another extra issues for guest ld.s.
>> [...]
>> I am not aware of any other possibility [virtualize DCR].
> 
> One of the options with a paravirtualized interface is to
> disallow hardware architecural corner-cases that are rarely
> used and very difficult/expensive to virtualize.  For
> example, in this case, one possibility is to state that
> an OS running on top of Xen must expect different behavior
> for the ld.s instruction when loading from memory locations
> pinned by a (virtual) TR.
> 
> I'm not saying this is the best answer for this problem,
> but it is a possibility.
> 
>> PS: Are you trying to run Windows ?
> 
> I have the same question... the above (ld.s from a TR-pinned
> location) doesn't occur in Linux does it?  Are you proposing
> a fix to a hypothetical problem, or does some other potential
> guest operating system make (frequent) use of this?
> 
> Thanks,
> Dan


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