Make hvm_fep available to non-debug build as well?

[Wei Liu <wei.liu2@citrix.com>]

Andrew and I had a short conversation on IRC about why hvm_fep is only
available to debug build. Here is what he said:

<andyhhp> liuw: because hvm_fep puts a very large attack surface back
          into the hypervisor
<andyhhp> I intoduced it originally to make it easy to test the
          instruction emulator without requiring a race condition between two
          vcpus
<andyhhp> so I guess paranoia is the underlying answer to your question
<andyhhp> there is nothing wrong in principle with making available in
          non-debug builds

I think I agree with him that in principle it should be possible to
make hvm_fep available to non-debug build. Andrew also suggested a
sync_console style warning, which I think makes sense.

If hvm_fep is enabled, we also should not provide any security support,
because the attack surface is widen. That should be reflected in the
warning message.

I think there is one major benefit for making hvm_fep available to
non-debug build:  user can replicate the exact setup in production and
use XTF to test if it is vulnerable to various issues.

My own reason for having hvm_fep for non-debug builds is that would make
integrating it with osstest much easier, and possibly require less
resources from osstest.  

I want XTF to run on every flight and gate xen-unstable. Remember that
we switch from debug to non-debug build  at some point during release
process -- if hvm_fep is only available to debug build, that would
require some more arrangement to run such tests.  We also essentially
test less for non-debug build, which is not very desirable.

Thoughts?

Wei.

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