Re: [PATCH 3/4] xen/arm: domain: Fix MISRA C 2012 Rule 8.7 violation

[Xenia Ragiadakou <burzalodowa@xxxxxxxxx>]

On 7/25/22 09:32, Jan Beulich wrote:
> On 24.07.2022 19:20, Xenia Ragiadakou wrote:
> > On 7/7/22 10:55, Jan Beulich wrote:
> > > On 07.07.2022 09:27, Xenia Ragiadakou wrote:
> > > > On 7/6/22 11:51, Jan Beulich wrote:
> > > > > On 06.07.2022 10:43, Xenia Ragiadakou wrote:
> > > > > > On 7/6/22 10:10, Jan Beulich wrote:
> > > > > > > On 05.07.2022 23:02, Xenia Ragiadakou wrote:
> > > > > > > > The function idle_loop() is referenced only in domain.c.
> > > > > > > > Change its linkage from external to internal by adding the storage-class
> > > > > > > > specifier static to its definitions.
> > > > > > > >
> > > > > > > > Since idle_loop() is referenced only in inline assembly, add the 'used'
> > > > > > > > attribute to suppress unused-function compiler warning.
> > > > > > > While I see that Julien has already acked the patch, I'd like to point
> > > > > > > out that using __used here is somewhat bogus. Imo the better approach
> > > > > > > is to properly make visible to the compiler that the symbol is used by
> > > > > > > the asm(), by adding a fake(?) input.
> > > > > > I 'm afraid I do not understand what do you mean by "adding a fake(?)
> > > > > > input". Can you please elaborate a little on your suggestion?
> > > > > Once the asm() in question takes the function as an input, the compiler
> > > > > will know that the function has a user (unless, of course, it finds a
> > > > > way to elide the asm() itself). The "fake(?)" was because I'm not deeply
> > > > > enough into Arm inline assembly to know whether the input could then
> > > > > also be used as an instruction operand (which imo would be preferable) -
> > > > > if it can't (e.g. because there's no suitable constraint or operand
> > > > > modifier), it still can be an input just to inform the compiler.
> > > > According to the following statement, your approach is the recommended one:
> > > > "To prevent the compiler from removing global data or functions which
> > > > are referenced from inline assembly statements, you can:
> > > > -use __attribute__((used)) with the global data or functions.
> > > > -pass the reference to global data or functions as operands to inline
> > > > assembly statements.
> > > > Arm recommends passing the reference to global data or functions as
> > > > operands to inline assembly statements so that if the final image does
> > > > not require the inline assembly statements and the referenced global
> > > > data or function, then they can be removed."
> > > >
> > > > IIUC, you are suggesting to change
> > > > asm volatile ("mov sp,%0; b " STR(fn) : : "r" (stack) : "memory" )
> > > > into
> > > > asm volatile ("mov sp,%0; b %1" : : "r" (stack), "S" (fn) : "memory" );
> > > Yes, except that I can't judge about the "S" constraint.
> > This constraint does not work for arm32. Any other thoughts?
> >
> > Another way, as Jan suggested, is to pass the function as a 'fake'
> > (unused) input.
> > I 'm suspecting something like the following could work
> > asm volatile ("mov sp,%0; b " STR(fn) : : "r" (stack), "X" (fn) : "memory")
> > What do you think?
> Well, yes, X should always be a fallback option. But I said already when
> you suggested S that I can't judge about its use, so I guess I'm the
> wrong one to ask. Even more so that you only say "does not work", without
> any details ...
The question is addressed to anyone familiar with arm inline assembly.
I added the arm maintainers as primary recipients to this email to make 
this perfectly clear.
When cross-compiling Xen on x86 for arm32 with
asm volatile ("mov sp,%0; b %1" : : "r" (stack), "S" (fn) : "memory" );
compilation fails with the error: impossible constraint in ‘asm'

--
Xenia