Re: [PATCH v4 1/3] xen/arm: add support for run_in_exception_handler()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.12.2020 10:15, Jürgen Groß wrote:
> On 14.12.20 10:03, Jan Beulich wrote:
>> On 14.12.2020 08:56, Juergen Gross wrote:
>>> Add support to run a function in an exception handler for Arm. Do it
>>> the same way as on x86 via a bug_frame.
>>>
>>> Unfortunately inline assembly on Arm seems to be less capable than on
>>> x86, leading to functions called via run_in_exception_handler() having
>>> to be globally visible.
>>
>> Could you extend on this? I don't understand what the relevant
>> difference is, from just looking at the changes.
> 
> The problem seems to be that a static symbol referenced from the inline
> asm seems not to silence the error that this static symbol isn't being
> used. On x86 the bug_frame is constructed using the %c modifier, which
> is not supported for Arm (at least gcc 7 used in my compile test
> complained), but seems to be enough for gcc on x86 to not complain.

But this isn't tied to %c not working on older gcc, is it? It looks
instead to be tied to you not specifying the function pointer as an
input in the asm(). The compiler would know the symbol is referenced
even if the input wasn't used at all in any of the asm()'s operands
(but of course it would be better to use the operand once you have
it, if it can be used in some way despite %c not being possible to
use).

Jan