Re: [Xen-devel] [PATCH] xen/public: arch-arm: Restrict the visibility of struct vcpu_guest_core_regs

[Artem Mygaiev <Artem_Mygaiev@xxxxxxxx>]

Hello Julien, Jan

On Wed, 2019-05-22 at 14:00 +0100, Julien Grall wrote:
> (+Artem)
> 
> Hi Jan,
> 
> On 22/05/2019 13:29, Jan Beulich wrote:
> > > > > On 22.05.19 at 14:20, <
> > > > > julien.grall@xxxxxxx
> > > > > > wrote:
> > > On 21/05/2019 10:55, Julien Grall wrote:
> > > > Hi Jan,
> > > > 
> > > > On 5/21/19 10:43 AM, Jan Beulich wrote:
> > > > > > > > On 21.05.19 at 11:35, <
> > > > > > > > julien.grall@xxxxxxx
> > > > > > > > > wrote:
> > > > > > 
> > > > > > On 5/21/19 10:26 AM, Jan Beulich wrote:
> > > > > > > > > > On 20.05.19 at 20:12, <
> > > > > > > > > > julien.grall@xxxxxxx
> > > > > > > > > > > wrote:
> > > > > > > > 
> > > > > > > >        As this is now Xen and tools only, I am
> > > > > > > > wondering whether the check on
> > > > > > > >        GNU_C is still necessary. I am happy to send a
> > > > > > > > follow-up patch (or fold
> > > > > > > >        in this one) if it can be removed.
> > > > > > > 
> > > > > > > I think this should be dropped if it can be without
> > > > > > > breaking any
> > > > > > > part of the build.
> > > > > > 
> > > > > > This is because all the tools are part of xen.git, right?
> > > > > 
> > > > > Right - no-one else is supposed to define __XEN_TOOLS__, or
> > > > > if anyone does, they're on their own.
> > > > 
> > > > Thanks for the information. I will do a full build check.
> > > 
> > > I thought about this again, long term there are an attempt to
> > > build xen with
> > > other compiler not necessarily supporting GNU C extension.
> > > While this would probably not be the only place that need to be
> > > reworked, we
> > > would have to revert part of this change. So I will not drop the
> > > #ifdef here.
> > 
> > Well, I don't know how it is for Arm, but on x86 we actually use
> > the
> > "extended" naming quite extensively, so building with a compiler
> > that doesn't support this extension is not really an option there.
> 
> For the Arm, I think only cpu_user_regs is using "extended" naming.
> It should be 
> possible to remove it without too much trouble here.
> 
> @Artem, is there any restriction to use anonymous union in functional
> safety?
> 

In general, unions are not allowed in safety regulated programming,
they always require a "deviation" - e.g. unions use for data packing is
usually accepted disregarding anonymous or not.

Couple of other things I wanted to mention:
1. all protective programming standards e.g. MISRA recommend reducing
visibility of functions and variables to reduce API surface ans thus
need for test coverage and systematic fault probability.
2. current implementation xen tools are very hard to use in safety for
many reasons, I hope to follow up on this soon...

 -- Artem
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