Re: [PATCH v3 3/4] xen/arm: switch ARM to use generic implementation of bug.h

[Julien Grall <julien@xxxxxxx>]

Hi,

On 01/03/2023 08:58, Oleksii wrote:
> On Tue, 2023-02-28 at 17:48 +0000, Julien Grall wrote:
> > Hi Oleksii,
> >
> > On 28/02/2023 15:09, Oleksii wrote:
> > > On Sat, 2023-02-25 at 16:49 +0000, Julien Grall wrote:
> > > > Hi Oleksii,
> > > >
> > > > On 24/02/2023 11:31, Oleksii Kurochko wrote:
> > > > > The following changes were made:
> > > > > * make GENERIC_BUG_FRAME mandatory for ARM
> > > > I have asked in patch #1 but will ask it again because I think
> > > > this
> > > > should be recorded in the commit message. Can you outline why it
> > > > is
> > > > not
> > > > possible to completely switch to the generic version?
> > > I haven't tried to switch ARM too because of comment regarding 'i'
> > > in
> > > <asm/bug.h>:
> > > /*
> > >    * GCC will not allow to use "i"  when PIE is enabled (Xen doesn't
> > > set
> > > the
> > >    * flag but instead rely on the default value from the compiler).
> > > So
> > > the
> > >    * easiest way to implement run_in_exception_handler() is to pass
> > > the
> > > to
> > >    * be called function in a fixed register.
> > >    */
> > I would expect this comment to be valid for any arch. So if there is
> > a
> > need to deal with PIE, then we would not be able to use "i" in the
> > BUG
> > frame.
> >
> > Note that we are now explicitly compiling Xen without PIE (see
> > Config.mk).
> Then it looks like some architectures isn't expected to be compiled
> with PIE. I mean that x86's bug.h is used 'i' and there is no any
> alternative version in case of PIE.
>
> If Xen should be compilable with PIE then we have to use ARM
> implementation of bug.h everywhere. ( based on comment about 'i' with
> PIE ).
The comment was added because until commit ecd6b9759919 ("Config.mk: 
correct PIE-related option(s) in EMBEDDED_EXTRA_CFLAGS") we would let 
the compiler to decide whether PIE should be enabled.
Now we are forcing -fno-pie for Xen on any architecture (the flag is 
added at the top-level in Config.mk).
> Now I am totally confused...
My point was not about using the Arm implementation everywhere. My point 
was that we disable even for Arm and therefore it is fine to use the 
common version.
If in the future we need to support PIE in Xen (I am not aware of any 
effort yet), then we could decide to update the common BUG framework. 
But for now, I don't think this is something you need to care in your 
series.
Cheers,

--
Julien Grall