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

[Julien Grall]

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@xxxxxxxxxx> 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@xxxxxxxx>
> > > 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