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RE: [Xen-ia64-devel] One unstablity in fast syscall path

To: "Tristan Gingold" <Tristan.Gingold@xxxxxxxx>, "Isaku Yamahata" <yamahata@xxxxxxxxxxxxx>
Subject: RE: [Xen-ia64-devel] One unstablity in fast syscall path
From: "Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>
Date: Mon, 19 Jun 2006 06:35:06 -0700
Cc: xen-ia64-devel <xen-ia64-devel@xxxxxxxxxxxxxxxxxxx>
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Thread-topic: [Xen-ia64-devel] One unstablity in fast syscall path
> > > On this point VTi may have a real advantage over 
> paravirtualization.
> >
> > Could you explain further?
> Yes.
> The virtualization handler has the opcode in GR25.
> I don't know the amount of PAL code involved, but the 
> hardware might provides 
> the opcode directly from the pipeline.  The VM doesn't have 
> to do the fecth, 
> which is time and cache consuming.

No the hardware does not provide the opcode directly, though
perhaps it will in future (post-Montecito) implementations.
I believe that PAL is doing the same thing that Xen is
doing to fetch the opcode, except since it is doing it
sooner, there is a higher probability that it will be
successful.  And I suspect that it may be doing the
"slow path" every time as it is probably not capable
of handling (and recovering from) TLB misses.  I'm not
sure I would call this an advantage.

Perhaps Intel can comment more here... I'm not sure what
info is public.


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