Re: ERROR: "__sync_icache_dcache" [drivers/xen/xen-privcmd.ko] undefined!

From: Catalin Marinas <catalin.marinas@arm.com>
To: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>, Arnd Bergmann <arnd@arndb.de>,
	Helmut Grohne <helmut.grohne@intenta.de>,
	Will Deacon <will.deacon@arm.com>,
	Russell King - ARM Linux <linux@armlinux.org.uk>,
	Paul Durrant <paul.durrant@citrix.com>,
	Ben Hutchings <ben@decadent.org.uk>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: ERROR: "__sync_icache_dcache" [drivers/xen/xen-privcmd.ko] undefined!
Date: Mon, 4 Mar 2019 22:28:51 +0000	[thread overview]
Message-ID: <20190304222851.GB36459@C02TF0J2HF1T.local> (raw)
In-Reply-To: <4b1af546-fb8c-f52a-1a39-ee265e7f410a@oracle.com>

On Mon, Mar 04, 2019 at 05:04:10PM -0500, Boris Ostrovsky wrote:
> On 12/18/18 8:21 AM, Catalin Marinas wrote:
> > On Tue, Dec 18, 2018 at 10:09:09AM +0000, Russell King wrote:
> >> On Tue, Dec 18, 2018 at 10:36:32AM +0100, Helmut Grohne wrote:
> >>> During a compile test of v4.20-rc7 for armhf (cross build), I
> >>> encountered:
> >>>
> >>> ERROR: "__sync_icache_dcache" [drivers/xen/xen-privcmd.ko] undefined!
> >>>
> >>> Some searching brought up c5157101e77 ("arm64: mm: Export
> >>> __sync_icache_dcache() for xen-privcmd"), which fixes a very similar
> >>> problem for arm64. Is that commit maybe also needed for arm?
> >> I would rather not export that function for drivers to abuse.
> 
> (I am reviving this thread because Arnd submitted a patch to work around
> this issue
> https://lore.kernel.org/lkml/20190304204826.2414365-1-arnd@arndb.de/ )
> 
> Even when the same routine in arm64 is exported for this very reason?
> 
> The patch above it going into a lot of trouble to make things work so I
> am hoping this can be re-considered.

It's Russell's decision whether to export this or not for arch/arm/. As
I said, I don't really see drivers abusing this since it doesn't help
with DMA cache maintenance (it only cleans to the PoU rather than PoC as
needed for DMA).

Now, the actual question is whether set_pte_at() is allowed to be called
from loadable modules. If yes, then __sync_icache_dcache() needs to be
exported. Otherwise, Arnd's patch looks fine. Grep'ing (5.0-rc3)
drivers/ for set_pte_at() I can only see 4 instances: one in
gpu/drm/i915/ which is not compiled as a module and three in xen/. Of
the latter, one is x86 only, the second is built-in and the third is
fixed by Arnd's patch.

-- 
Catalin

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