Re: [PATCH for 4.6] x86/hvm.c: fix regression in guest destruction

[George Dunlap <george.dunlap@citrix.com>]

On 07/30/2015 10:32 AM, Ian Campbell wrote:
> On Thu, 2015-07-30 at 10:21 +0100, Ian Campbell wrote:
> 
>> which I have applied with. I still don't think the commit message is very
>> satisfactory, but I'm not maintainer of any of this code so meh.
> 
> For the benefit of the archives perhaps someone could explain why gating a
> per-vcpu teardown on a host level feature setting is correct?
> 
> In particular what ensures that altp2m_vcpu_initialise has been called,
> given that this is only called from HVMOP_altp2m_set_domain_state. What
> happens if that HVMOP is never touched?
> 
> Do things work both for altp2m disabled on the Xen command line and
> disabled/enabled in the guest config? If so how?
> 
> Also how come HVMOP_altp2m_set_domain_state does not have a
> hvm_altp2m_supported check?

So this was all acked & stuff before I had much of a chance to comment
on it, but on my to-do list for 4.7 is to rework a lot of the
initialization / teardown stuff.  In particular:

- Always and only check for whether something has been initialized
(e.g., non-NULL, non-INVALID_MFN) before tearing it down

- Do *all* of the initialization for both altp2m and nestedhvm when
they're actually enabled for the domain, rather than doing a bunch of
the initialization unconditionally up front.

This is all part of the "technical debt" we were talking about when we
considered giving it a freeze exception.

 -George