Re: [PATCH v1 02/13] xen/arm: introduce a special domain DOMID_SHARED

[Julien Grall <julien.grall@gmail.com>]

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Hi Penny,

Sorry for the formatting.

On Fri, 15 Apr 2022 at 09:53, Penny Zheng <Penny.Zheng@arm.com> wrote:

> Hi jan
>
> > -----
> > > +#ifdef CONFIG_STATIC_SHM
> > > +static void __init setup_shared_domain(void) {
> > > +    /*
> > > +     * Initialise our DOMID_SHARED domain.
> > > +     * This domain owns statically shared pages when owner domain is
> not
> > > +     * explicitly defined.
> > > +     */
> > > +    dom_shared = domain_create(DOMID_SHARED, NULL, CDF_directmap);
> > > +    if ( IS_ERR(dom_shared) )
> > > +        panic("Failed to create d[SHARED]: %ld\n",
> > > +PTR_ERR(dom_shared));
> >
> > I don't think this should be a panic - the system ought to be able to
> come up
> > fine, just without actually using this domain. After all this is an
> optional
> > feature which may not actually be used.
> >
> > Also, along the lines of what Stefano has said, this setting up of the
> domain
> > would also better live next to where the other special domains are set
> up. And
> > even if it was to remain here, ...
> >
>
> The reason why I place the setting up here is that DOMID_SHARED needs to
> map
> pre-configured static shared memory in its p2m table, so it must be set up
> after system P2M initialization(setup_virt_paging()).
> setup_system_domains()
> is called before system P2M initialization on xen/arch/arm/setup.c, which
> can't meet the requirement.


AFAIU, DOM_SHARED (or whatever you want to call it) will never run and the
GFN will always be equal to the MFN. So it sounds to me that creating a P2M
is a bit over the top. Instead, we should have a special case in
get_page_from_gfn like we did for DOM_XEN.

Cheers,
-- 
Julien Grall

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