Re: [Xen-devel] [PATCH] ARM: xen: unexport HYPERVISOR_platform_op function

From: Julien Grall <julien.grall@arm.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>, Arnd Bergmann <arnd@arndb.de>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Emil Velikov <emil.l.velikov@gmail.com>,
	Russell King <linux@armlinux.org.uk>,
	Denis Efremov <efremov@linux.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [Xen-devel] [PATCH] ARM: xen: unexport HYPERVISOR_platform_op function
Date: Sat, 7 Sep 2019 11:05:45 +0100	[thread overview]
Message-ID: <95dbd972-fe78-d0ca-f7b4-1a6bdd418eab@arm.com> (raw)
In-Reply-To: <bda2a05b-e2d0-feee-761b-88deeeac2449@citrix.com>

Hi Andrew,

On 9/6/19 6:20 PM, Andrew Cooper wrote:
> On 06/09/2019 17:00, Arnd Bergmann wrote:
>> On Fri, Sep 6, 2019 at 5:55 PM Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>>> On 06/09/2019 16:39, Arnd Bergmann wrote:
>>>> HYPERVISOR_platform_op() is an inline function and should not
>>>> be exported. Since commit 15bfc2348d54 ("modpost: check for
>>>> static EXPORT_SYMBOL* functions"), this causes a warning:
>>>>
>>>> WARNING: "HYPERVISOR_platform_op" [vmlinux] is a static EXPORT_SYMBOL_GPL
>>>>
>>>> Remove the extraneous export.
>>>>
>>>> Fixes: 15bfc2348d54 ("modpost: check for static EXPORT_SYMBOL* functions")
>>>> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>>> Something is wonky.  That symbol is (/ really ought to be) in the
>>> hypercall page and most definitely not inline.
>>>
>>> Which tree is that changeset from?  I can't find the SHA.
>> This is from linux-next, I think from the kbuild tree.
> 
> Thanks.
> 
> Julien/Stefano: Why are any of these hypercalls out-of-line?  ARM
> doesn't use the hypercall page, and there is no argument translation
> (not even in arm32 as there are no 5-argument hypercalls declared).

I am not sure how the hypercall page makes things different. You still 
have to store the arguments in the correct register so...

> 
> They'd surely be easier to implement with a few static inlines and some
> common code, than to try and replicate the x86 side hypercall_page
> interface ?

... I don't think they will be easier to implement with a few static 
inlines. The implementation will likely end up to be similar to 
arch/x86/asm/xen/hypercall.h.

Furthermore, one of the downside of per-arch static inline is it is more 
difficult to ensure the prototype match for all the architectures. 
Although, it might be possible to make them common by only requesting 
per-arch to implement HYPERCALL_N(...).

So I think the code is better as it is.

While looking at the code, I also realized that the implementation of 
HYPERCALL_dm_op might be incorrect for Arm32. Similarly do privcmd call, 
I think dm_op call should enable user access as they will be used by 
userspace.

We don't use dm_op on Arm so far, hence why I think this was unnoticed. 
I will see if I can reproduce it and send a patch.

Cheers,

-- 
Julien Grall