Re: [XEN PATCH] xen/mem_access: address violations of MISRA C: 2012 Rule 8.4

[Alessandro Zucchelli <alessandro.zucchelli@xxxxxxxxxxx>]

On 2024-05-03 11:32, Julien Grall wrote:
> Hi,
>
> On 03/05/2024 08:09, Alessandro Zucchelli wrote:
> > On 2024-04-29 17:58, Jan Beulich wrote:
> > > On 29.04.2024 17:45, Alessandro Zucchelli wrote:
> > > > Change #ifdef CONFIG_MEM_ACCESS by OR-ing defined(CONFIG_ARM),
> > > > allowing asm/mem_access.h to be included in all ARM build 
> > > > configurations.
> > > > This is to address the violation of MISRA C: 2012 Rule 8.4 which 
> > > > states:
> > > > "A compatible declaration shall be visible when an object or 
> > > > function
> > > > with external linkage is defined". Functions p2m_mem_access_check
> > > > and p2m_mem_access_check_and_get_page when CONFIG_MEM_ACCESS is not
> > > > defined in ARM builds don't have visible declarations in the file
> > > > containing their definitions.
> > > >
> > > > Signed-off-by: Alessandro Zucchelli 
> > > > <alessandro.zucchelli@xxxxxxxxxxx>
> > > > ---
> > > >  xen/include/xen/mem_access.h | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/xen/include/xen/mem_access.h 
> > > > b/xen/include/xen/mem_access.h
> > > > index 87d93b31f6..ec0630677d 100644
> > > > --- a/xen/include/xen/mem_access.h
> > > > +++ b/xen/include/xen/mem_access.h
> > > > @@ -33,7 +33,7 @@
> > > >   */
> > > >  struct vm_event_st;
> > > >
> > > > -#ifdef CONFIG_MEM_ACCESS
> > > > +#if defined(CONFIG_MEM_ACCESS) || defined(CONFIG_ARM)
> > > >  #include <asm/mem_access.h>
> > > >  #endif
> > > This doesn't look quite right. If Arm supports mem-access, why would 
> > > it
> > > not set MEM_ACCESS=y? Whereas if it's only stubs that Arm supplies, 
> > > then
> > > those would better move here, thus eliminating the need for a 
> > > per-arch
> > > stub header (see what was e.g. done for numa.h). This way RISC-V and 
> > > PPC
> > > (and whatever is to come) would then be taken care of as well.
> > ARM does support mem-access, so I don't think this is akin to the 
> > changes done to handle numa.h.
> > ARM also allows users to set MEM_ACCESS=n (e.g. 
> > xen/arch/arm/configs/tiny64_defconfig) and builds just fine; however, 
> > the implementation file mem_access.c is compiled unconditionally in 
> > ARM's makefile, hence why the violation was spotted.
> > This is a bit unusual, so I was also hoping to get some feedback from 
> > mem-access maintainers as to why this discrepancy from x86 exists. I 
> > probably should have also included some ARM maintainers as well, so 
> > I'm going to loop them in now.
> > An alternative option I think is to make the compilation of arm's 
> > mem_access.c conditional on CONFIG_MEM_ACCESS (as for x86/mm and 
> > common).
> I can't think of a reason to have mem_access.c unconditional compiled 
> in. So I think it should be conditional like on x86.
Hi,
attempting to build ARM with a configuration where MEM_ACCESS=n and 
mem_access.c is conditioned on CONFIG_MEM_ACCESS results in a fail as 
there are other pieces of code that call some mem_access.c functions 
(p2m_mem_access_check_and_get_page, p2m_mem_access_check).
In a Matrix chat Julien was suggesting adding stubs for the functions 
for this use case.
--
Alessandro Zucchelli, B.Sc.

Software Engineer, BUGSENG (https://bugseng.com)