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

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
.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?


Julien Grall



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