Re: [Xen-devel] Requesting a freeze exception for vm_event memory introspection helpers

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Jul 13, 2015 at 05:10:21PM +0300, Razvan Cojocaru wrote:
> On 07/13/2015 04:51 PM, Wei Liu wrote:
> > On Mon, Jul 13, 2015 at 10:35:44AM +0300, Razvan Cojocaru wrote:
> >> Hello,
> >>
> >> I'd like to ask for a freeze exception for the "vm_event memory
> >> introspection helpers" series.
> >>
> >> [PATCH 1/3] xen/mem_access: Support for memory-content hiding
> >> [PATCH 2/3] xen/vm_event: Support for guest-requested events
> >> [PATCH 3/3] xen/vm_event: Deny register writes if refused by
> >> vm_event reply
> >>
> >> All patches have been acked by at least one person (though patch 1 is
> >> still under some discussion).
> >>
> >> 1. Benefits of the series making it in this release:
> >>
> >> * Probably the most important benefit is that 4.6 development has been
> >> very open to refactoring vm_events, and patch 3/3 makes vm_events behave
> >> in a consistent manner (all register-write vm_events are pre-write events).
> >>
> >> * There are 3rd parties interested in these features (Tamas, for
> >> example, has already expressed interest in uses of patch 1/1).
> >>
> > 
> > These aren't really good arguments for getting this in for 4.6. It would
> > be easy for you and Tamas to carry three patches for a while.
> > 
> > What are the impact to end users (end users -- meaning either system
> > administrator that sets up Xen and other developers who want to use the
> > new interface you introduce)?  Does this series consist of the last
> > missing pieces of a feature?
> 
> Not just missing pieces of a feature, the guest-requested (formerly
> VMCALL) vm_event patch is crucial for memory introspection, and without
> the first patch it's very difficult to resume monitoring Windows guests
> that have been put in hybernate-mode (instead of powered off).
> 

Right, so with these three patches, memory introspection is a complete
feature (as in, to cover most use cases)? Or is it still a work in
progress even after these patches are merged?

> I don't follow the impact question, sorry. If an administrator doesn't
> use these features, the impact (in terms of speed and resources) should
> be next to nothing.
> 

OK, let me be specific. If I'm to build a product by either using Xen as
out-of-box virtualisation platform or building my own software on top of
Xen, would it make any *big* difference if I have your three patches?

The "big difference" is the impact (benefit) I'm looking at, from users'
point of view.

> If a freeze exception for the whole series is not an option at this
> point, would it be appropriate to just drop patches requiring more
> review and get the more acked ones in?
> 

I'm not saying it's not possible. I'm trying to make sense of what is
going on at the moment.

Maybe other maintainers can help enlighten me and express their
preference? And I guess with your maintainer's hat on it would be a
"yes, let's merge them" from you. :-)

Wei.

> 
> Thanks,
> Razvan

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