Re: [PATCH v6 1/3] xen/arm: add support for run_in_exception_handler()

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 21/01/2021 07:55, Jan Beulich wrote:
> On 20.01.2021 19:20, Julien Grall wrote:
> > On 16/01/2021 10:33, Juergen Gross wrote:
> > > Add support to run a function in an exception handler for Arm. Do it
> > > as on x86 via a bug_frame, but pass the function pointer via a
> > > register (this needs to be done that way, because inline asm support
> > > for 32-bit Arm lacks the needed functionality to reference an
> > > arbitrary function via the bugframe).
> > I was going to commit the series, but then realized the commit message
> > and comment needs some tweaking because technically GCC is supporting
> > 'i' (I managed to get it working with -fno-pie).
> >
> > So how about:
> >
> > "This is needed to be done that way because GCC complains about invalid
> > constraint when using a function pointer with "i" and PIE is enabled
> > (default on most of GCC shipped with distro). Clang happily accepts it,
> > so it may be a bug in GCC."
> May I ask why you think it's a bug in gcc? I'd rather consider it
> a bug in clang. Taking the address of a symbol doesn't yield a
> constant in PIC or PIE code, aiui.
I consider it a bug because we were using it as:

.pushsection .bug.frame
2b:
.long (%0 - 2b)

So I expect the compiler to be able to find the displacement in both 
cases as we don't need to know the exact address.
I think Clang is just clever enough to figure out we want a displacement.

Do you have a suggestion of constraint that could resolve the issue?

Cheers,

--
Julien Grall