linux-kernel - Re: [Xen-devel] [PATCH] ARM: xen: unexport HYPERVISOR_platform

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Date:   Sat, 7 Sep 2019 11:05:45 +0100
From:   Julien Grall <julien.grall@....com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Arnd Bergmann <arnd@...db.de>
Cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Emil Velikov <emil.l.velikov@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Denis Efremov <efremov@...ux.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [PATCH] ARM: xen: unexport HYPERVISOR_platform_op
 function

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@...rix.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@...db.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