by Stefano Stabellini

[permalink] [raw]

Subject: Re: [PATCH V3 4/6] xen/unpopulated-alloc: Add mechanism to use Xen resource

On Thu, 9 Dec 2021, Oleksandr wrote:
> On 08.12.21 01:36, Stefano Stabellini wrote:
> Hi Stefano
>
> > On Thu, 25 Nov 2021, Oleksandr wrote:
> > > > > Please note the following:
> > > > > for V3 arch_xen_unpopulated_init() was moved to init() as was agreed
> > > > > and gained __init specifier. So the target_resource is initialized
> > > > > there.
> > > > >
> > > > > With current patch series applied if CONFIG_XEN_UNPOPULATED_ALLOC
> > > > > is enabled:
> > > > >
> > > > > 1. On Arm, under normal circumstances, the
> > > > > xen_alloc_unpopulated_pages()
> > > > > won't be called “before” arch_xen_unpopulated_init(). It will only be
> > > > > called "before" when either ACPI is in use or something wrong happened
> > > > > with DT (and we failed to read xen_grant_frames), so we fallback to
> > > > > xen_xlate_map_ballooned_pages() in
> > > > > arm/xen/enlighten.c:xen_guest_init(),
> > > > > please see "arm/xen: Switch to use gnttab_setup_auto_xlat_frames() for
> > > > > DT"
> > > > > for details. But in that case, I think, it doesn't matter much whether
> > > > > xen_alloc_unpopulated_pages() is called "before" of "after"
> > > > > target_resource
> > > > > initialization, as we don't have extended regions in place the
> > > > > target_resource
> > > > > will remain invalid even after initialization, so
> > > > > xen_alloc_ballooned_pages()
> > > > > will be used in both scenarios.
> > > > >
> > > > > 2. On x86, I am not quite sure which modes use unpopulated-alloc
> > > > > (PVH?),
> > > > > but it looks like xen_alloc_unpopulated_pages() can (and will) be
> > > > > called
> > > > > “before” arch_xen_unpopulated_init().
> > > > > At least, I see that xen_xlate_map_ballooned_pages() is called in
> > > > > x86/xen/grant-table.c:xen_pvh_gnttab_setup(). According to the
> > > > > initcall
> > > > > levels for both xen_pvh_gnttab_setup() and init() I expect the former
> > > > > to be called earlier.
> > > > > If it is true, the sentence in the commit description which mentions
> > > > > that “behaviour on x86 is not changed” is not precise. I don’t think
> > > > > it would be correct to fallback to xen_alloc_ballooned_pages() just
> > > > > because we haven’t initialized target_resource yet (on x86 it is just
> > > > > assigning it iomem_resource), at least this doesn't look like an
> > > > > expected
> > > > > behaviour and unlikely would be welcome.
> > > > >
> > > > > I am wondering whether it would be better to move
> > > > > arch_xen_unpopulated_init()
> > > > > to a dedicated init() marked with an appropriate initcall level
> > > > > (early_initcall?)
> > > > > to make sure it will always be called *before*
> > > > > xen_xlate_map_ballooned_pages().
> > > > > What do you think?
> > >    ... here (#2). Or I really missed something and there wouldn't be an
> > > issue?
> > Yes, I see your point. Yeah, it makes sense to make sure that
> > drivers/xen/unpopulated-alloc.c:init is executed before
> > xen_pvh_gnttab_setup.
> >
> > If we move it to early_initcall, then we end up running it before
> > xen_guest_init on ARM. But that might be fine: it looks like it should
> > work OK and would also allow us to execute xen_xlate_map_ballooned_pages
> > with target_resource already set.
> >
> > So I'd say go for it :)
>
>
> Thank you for the confirmation! In order to be on the safe side, I would
> probably leave drivers/xen/unpopulated-alloc.c:init as is, I mean with current
> subsys initcall level (it expects the extra memory regions to be already
> filled)
> and create a separate unpopulated_init() to put arch_xen_unpopulated_init()
> into. Something like the following:
>
> static int __init unpopulated_init(void)
> {
>     int ret;
>
>     if (!xen_domain())
>         return -ENODEV;
>
>     ret = arch_xen_unpopulated_init(&target_resource);
>     if (ret) {
>         pr_err("xen:unpopulated: Cannot initialize target resource\n");
>         target_resource = NULL;
>     }
>
>     return ret;
> }
> early_initcall(unpopulated_init);

Sounds good