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

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Date:   Tue, 7 Dec 2021 15:36:53 -0800 (PST)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Oleksandr <olekstysh@...il.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>, Julien Grall <julien@....org>
Subject: Re: [PATCH V3 4/6] xen/unpopulated-alloc: Add mechanism to use Xen
 resource

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 :)