RE: [Xen-ia64-devel] Xen is now runnnig at Bull

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

Resend to correct typo

Magenheimer, Dan (HP Labs Fort Collins) wrote:
> Actually, it is conceivable that a single processor
> machine would want to support more than 64 VMs so
> it is very likely that a 48 processor machine would.
> The limit of 64 is on the number of domains that
> might be active, not necessarily simultaneously active.
> 
> I don't think rid virtualization helps here as there
> are only 2**24 rids available and every domain will
> use as many as it is given.  And there's no way to garbage
> collect rids from a domain. So -- I think -- the only ways
> to support more than 64 domains are:
> 1) Give each domain fewer than 2**18 rids, or
> 2) Flush the TLB whenever "necessary" when switching domains
> 
> The frequency of "necessary" could be reduced by using
> a processor affinity hierarchy of some kind.  I suppose
> this is a form of rid virtualization, but is more like
> an extension (e.g. like PAE).
> 
> Dan
> 
RID virtualization targets to solve 2 problems:
1: VHPT locality issue. 
       Matt's mangling algorithm may solve problem for single VM case if
all itanium processor use same hash algorithm, but still has problem in
multiple VM
situation. (The high bits of different VM doesn't participate the hash
algorithm)
2: guest get full 24 bits of rid.

The way to garbage rid is not very complicate. :-(

When the guest rid is not used anymore, the physical rid should be
garbaged.
A simple way to determine when the guest rid is not used can be done by
looking at the VHPT table and machine TLB side. 
(Yes, a reference count needs to be introduced for each guest rid to
track the
entries in VHPT, and a special indicator in guest tlb to indicating if
there are
entries in physical TLB)


Is it time for me to share what we thought of for rid virtualization in
much details?

Eddie

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