RE: [PATCH 1/5] xen/common: introduce a new framework for save/restore of 'domain' context

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Julien Grall <julien@xxxxxxx>
> Sent: 06 April 2020 10:08
> To: paul@xxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: 'Andrew Cooper' <andrew.cooper3@xxxxxxxxxx>; 'George Dunlap' 
> <george.dunlap@xxxxxxxxxx>; 'Ian
> Jackson' <ian.jackson@xxxxxxxxxxxxx>; 'Jan Beulich' <jbeulich@xxxxxxxx>; 
> 'Stefano Stabellini'
> <sstabellini@xxxxxxxxxx>; 'Wei Liu' <wl@xxxxxxx>
> Subject: Re: [PATCH 1/5] xen/common: introduce a new framework for 
> save/restore of 'domain' context
> 
> Hi Paul,
> 
> On 06/04/2020 09:27, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Julien Grall <julien@xxxxxxx>
> >> Sent: 03 April 2020 18:24
> >> To: paul@xxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx
> >> Cc: 'Andrew Cooper' <andrew.cooper3@xxxxxxxxxx>; 'George Dunlap' 
> >> <george.dunlap@xxxxxxxxxx>; 'Ian
> >> Jackson' <ian.jackson@xxxxxxxxxxxxx>; 'Jan Beulich' <jbeulich@xxxxxxxx>; 
> >> 'Stefano Stabellini'
> >> <sstabellini@xxxxxxxxxx>; 'Wei Liu' <wl@xxxxxxx>
> >> Subject: Re: [PATCH 1/5] xen/common: introduce a new framework for 
> >> save/restore of 'domain' context
> >>
> >> Hi Paul,
> >>
> >> On 03/04/2020 16:55, Paul Durrant wrote:
> >>>> -----Original Message-----
> >>> [snip]
> >>>>> +
> >>>>> +#include <xen/save.h>
> >>>>> +
> >>>>> +struct domain_context {
> >>>>> +    bool log;
> >>>>> +    struct domain_save_descriptor desc;
> >>>>> +    domain_copy_entry copy;
> >>>>
> >>>> As your new framework is technically an extension of existing one, it
> >>>> would be good to explain why we diverge in the definitions.
> >>>>
> >>>
> >>> I don't follow. What is diverging? I explain in the commit comment that 
> >>> this is a parallel
> >> framework. Do I need to justify why it is not a carbon copy of the HVM one?
> >>
> >> Well, they are both restoring/saving guest state. The only difference is
> >> the existing one is focusing on HVM state.
> >>
> >> So it would make sense long term to have only one hypercall and tell
> >> what you want to save. In fact, some of the improvement here would
> >> definitely make the HVM one nicer to use (at least in the context of LU).
> >>
> >
> > I guess we could move the HVM save records over to the new framework, but 
> > it works for the moment so
> I don't want to bring it into scope now.
> 
> And I agree, hence why I say "long term" :).
> 
> >
> >>   From the commit message, it is not clear to me why a new framework and
> >> why the infrastructure is at the same time different but not.
> >>
> >
> > An alternative would be to move the HVM save code into common code and then 
> > try to adapt it. I think
> that would result in more code churn and ultimately be harder to review. The 
> extra infrastructure
> introduced here is fairly minimal and, for the moment, only targeting PV 
> state. As I said above
> there's nothing stopping the HVM records being ported over later once any 
> initial issues have been
> shaken out.
> 
> Code churn is always going to necessary one day or another if we want to
> consolidate the two.
> 
> Having two frameworks is not without risks. There are a few unknown to
> be answered:
>    * Is there any dependency between the two? If yes, what is the ordering?

There isn't any dependency at the moment so need I say anything? If an ordering 
dependency is introduced by a future patch then surely that would be time to 
call it out.

>    * The name of the hypercall does not say anything about "PV". So a
> contributor could think it would be fine to save/restore new HVM state
> in the new generic hypercall. Is it going to be an issue? If so, how do
> we prevent it?

The commit message says:

"Domain context is state held in the hypervisor that does not come under
the category of 'HVM state' but is instead 'PV state' that is common
between PV guests and enlightened HVM guests (i.e. those that have PV
drivers) such as event channel state, grant entry state, etc."

Do you think this should also appear in a comment? Do we really care? Nothing 
fundamentally prevents the mechanism being used for HVM state, but that may 
introduce an ordering dependency.

>    * Are we going to deprecate the existing framework?
> 

There is no intention as yet.

> I am not suggesting we should not go with two frameworks, but the
> reasons and implications are not clear to me. Hence, why I think the
> commit message should be expanded with some rationale.
> 

Ok, I can add a paragraph to try to explain a little more.

  Paul