RE: [PATCH v4 2/8] xen/arm: allocate static shared memory to the default owner dom_io

[Penny Zheng <Penny.Zheng@xxxxxxx>]

Hi Jan

Sorry about the late reply, got sidetracked a few weeks.

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: Wednesday, May 18, 2022 2:36 PM
> To: Penny Zheng <Penny.Zheng@xxxxxxx>
> Cc: Wei Chen <Wei.Chen@xxxxxxx>; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; Julien Grall <julien@xxxxxxx>; Bertrand Marquis
> <Bertrand.Marquis@xxxxxxx>; Volodymyr Babchuk
> <Volodymyr_Babchuk@xxxxxxxx>; Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx>; George Dunlap <george.dunlap@xxxxxxxxxx>;
> Wei Liu <wl@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH v4 2/8] xen/arm: allocate static shared memory to the
> default owner dom_io
> 
> On 18.05.2022 05:14, Penny Zheng wrote:
> > Hi Jan
> >
> >> -----Original Message-----
> >> From: Jan Beulich <jbeulich@xxxxxxxx>
> >> Sent: Wednesday, May 18, 2022 12:01 AM
> >> To: Penny Zheng <Penny.Zheng@xxxxxxx>
> >> Cc: Wei Chen <Wei.Chen@xxxxxxx>; Stefano Stabellini
> >> <sstabellini@xxxxxxxxxx>; Julien Grall <julien@xxxxxxx>; Bertrand
> >> Marquis <Bertrand.Marquis@xxxxxxx>; Volodymyr Babchuk
> >> <Volodymyr_Babchuk@xxxxxxxx>; Andrew Cooper
> >> <andrew.cooper3@xxxxxxxxxx>; George Dunlap
> >> <george.dunlap@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>;
> >> xen-devel@xxxxxxxxxxxxxxxxxxxx
> >> Subject: Re: [PATCH v4 2/8] xen/arm: allocate static shared memory to
> >> the default owner dom_io
> >>
> >> On 17.05.2022 11:05, Penny Zheng wrote:
> >>> --- a/xen/common/domain.c
> >>> +++ b/xen/common/domain.c
> >>> @@ -780,6 +780,11 @@ void __init setup_system_domains(void)
> >>>       * This domain owns I/O pages that are within the range of the
> page_info
> >>>       * array. Mappings occur at the priv of the caller.
> >>>       * Quarantined PCI devices will be associated with this domain.
> >>> +     *
> >>> +     * DOMID_IO could also be used for mapping memory when no explicit
> >>> +     * domain is specified.
> >>> +     * For instance, DOMID_IO is the owner of memory pre-shared among
> >>> +     * multiple domains at boot time, when no explicit owner is 
> >>> specified.
> >>>       */
> >>>      dom_io = domain_create(DOMID_IO, NULL, 0);
> >>>      if ( IS_ERR(dom_io) )
> >>
> >> I'm sorry: The comment change is definitely better now than it was,
> >> but it is still written in a way requiring further knowledge to
> >> understand what it talks about. Without further context, "when no
> >> explicit domain is specified" only raises questions. I would have
> >> tried to make a suggestion, but I can't really figure what it is that you 
> >> want
> to get across here.
> >
> > How about I only retain the "For instance, xxx" and make it more in details.
> > "
> > DOMID_IO is also the default owner of memory pre-shared among multiple
> > domains at boot time, when no explicit owner is specified with "owner"
> > property in static shared memory device node. See section
> > docs/misc/arm/device-tree/booting.txt: Static Shared Memory for more
> details.
> > "
> 
> This reads quite a bit better. Yet I continue to be puzzled about the apparent
> conflict of "pre-shared" and "no explicit owner": How can memory be (pre-
> )shared when the owner isn't known? Shouldn't all memory have an owner?
> Or alternatively if this sharing model doesn't require ownership, shouldn't 
> all
> shared memory be owned by DomIO? In any event, to leave such details out of
> here, perhaps the comment could consist of just the first part of what you
> wrote, ending at where the first comma is?
> 

We have a short discussion about the memory ownership on my design link(
https://lore.kernel.org/all/a50d9fde-1d06-7cda-2779-9eea9e1c0134@xxxxxxx/T/)
, we have user cases for both scenario.

Ok, I will modify the comment and only keep
"
DOMID_IO is also the default owner of memory pre-shared among multiple
domains at boot time.
"
 
> Jan