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Re: [Fwd: Re: [Xen-devel] [RFC] XI Shadow Page Table Mechanism]

Robert Phillips wrote:

    This isn't supported currently?  Since an HVM must go through 16
    bit, 32
    bit, and 64 bit mode to boot up, how can we start more than one
    guest at
    a time currently if this doesn't already work?

Ed Smith's daily test results show there have been problems with SMP. We haven't diagnosed these problems but, from reading the pre-XI shadow code, it's not clear how it copes with multiple VCPUs in the same domain running in different modes.

There's only a short period of time when this would be happening right? During boot up?

    - how do you deal with large pages within the hypervisor?  do you
coalesce or just hope there is contiguous pages available?

We just hope that contiguous pages are available. However we allocate pages for the guest in large extents to maximize this likelihood. In practice this is very effective for guests created soon after boot time. There may be fragmentation problems later.

Any ideas about how to deal with this long term? Large page support would also be useful for PV domains. There was a lot of people at the last summit that were interested in this...

Thanks for the responses,

Anthony Liguori

    - what is the performance benefit in saving the shadow pages for each
    domain?  there's clearly a memory trade-off here so understanding the
    performance gain seems important.

The performance benefit appears to be substantial but we have not done a thorough study yet.

    - OOM can be dealt with in the existing code by just invalidating
    existing mappings to free up pages.  what advantages do your approach
    have to this?  (i realize we don't do this today but in theory, we

Ultimately XI deals with OOM by tearing down cached shadow pages, just as you say. But it uses LRU to pick the victims.

    Interesting stuff.  I'm eager to see the code.


    Anthony Liguori

    > Thanks,
    > -b
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    Ben Thomas                                         Virtual Iron
    <mailto:bthomas@xxxxxxxxxxxxxxx>                            Tower
    1, Floor 2
    978-849-1214                                       900 Chelmsford
                                                        Lowell, MA 01851

Robert S. Phillips                          Virtual Iron Software
rsp.vi.xen@xxxxxxxxxxxxxxx <mailto:rsp.vi.xen@xxxxxxxxxxxxxxx> Tower 1, Floor 2
978-849-1220                                 900 Chelmsford Street
Lowell, MA 01851

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