Re: Xen as a kernel module

[Jacob Gorm Hansen <jacobg@diku.dk>]

Neugebauer, Rolf wrote:

> However, I think it would be foolish to remove the ability to run
> separate driver domains from Xen as it doesn't seem to add significant
> overhead (over running them in Dom0) and it has the potential of being
> extremely useful. You already mentioned the shipping of device drivers,
> there might also some interesting security applications. We can
> currently already provide some degree of isolation against device driver
> bugs and for the DMA issue there are a number of known and existing
> solutions (see eg the L4 OSDI device driver paper). The grant-table
> design (unfortunately not fully implemented yet) with some form of
> IO-MMU should provide the rest of the isolation.

First of all, my suggestion was to have this as a configuration option, 
or even a separate implementation, providing just the ability to host 
domUs. I am sure this is doable, as it seems to be what coLinux is doing 
with some success. I was not suggestion throwing away existing 
functionality, but to optimize for the common case, i.e. a single Linux 
providing all drivers.

Secondly, there have been repeated reports on this list of people having 
problems with lower performance in domU than in dom0, perhaps due to 
cheap hardware, perhaps just due to misconfiguration, and the figures on 
the Xen website have not been updated to reflect what the actual 
situation is, so I guess nobody knows what the overhead will look like 
for a specific type of application. I would worry about MPI-style jobs, 
where you need both low latency and high bandwidth networking, and where 
you are likely to fully utilize the TLBs as well. I cannot see how 
performance does not get hurt in this situation, when Xen needs to flush 
the TLB for every interrupt, or alternatively needs to bundle 
interrupts, thus increasing latency.

Jacob


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