RE: [Xen-devel] Essay on an important Xen decision (long)

["Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>]

> > Not only possible, but normal.  If there is a TLB miss and
> > a VHPT (virtual hashed page table) miss, software fills
> > both the TLB and VHPT.
> >
> > Is that what you meant?
> 
> Yep, that's exactly what I thought happened :-)
> 
> IIRC, you said you don't bother with the guest VHPT, right?  
> So presumably you 
> reflect TLB misses to the guest and intercept its TLB fill 
> instruction, apply 
> the P2M translation, then add it to *Xen*'s VHPT and fill the 
> TLB correctly?
> 
> I know I've followed some of these discussions before, just a 
> bit rusty now ;-)

Exactly... except for one nice shortcut that Matt Chapman
added.  Since the VHPT is architected and the guest is
expecting that it may be walked, when Xen intercepts the
initial TLB miss, it can first look in the guest VHPT
to resolve the miss (and add it to Xen's VHPT and fill
the TLB) rather than reflect the TLB miss to the guest.
Only if the translation isn't found in the guest VHPT
(or if looking for it -- a user_access -- causes another
TLB miss), then the TLB miss is reflected to the guest.

Thus, guests have the benefit not only of the hardware TLB
and Xen's VHPT but also their own VHPT.

Dan

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