Re: [PATCH v2 3/6] xen/riscv: introduce function for physical offset calculation

[Oleksii <oleksii.kurochko@xxxxxxxxx>]

On Fri, 2023-07-07 at 10:17 +0100, Julien Grall wrote:
> 
> 
> On 07/07/2023 10:12, Oleksii wrote:
> > On Thu, 2023-07-06 at 13:18 +0200, Jan Beulich wrote:
> > > On 19.06.2023 15:34, Oleksii Kurochko wrote:
> > > > --- a/xen/arch/riscv/riscv64/head.S
> > > > +++ b/xen/arch/riscv/riscv64/head.S
> > > > @@ -29,6 +29,8 @@ ENTRY(start)
> > > >   
> > > >           jal     reset_stack
> > > >   
> > > > +        jal     calc_phys_offset
> > > > +
> > > >           tail    start_xen
> > > >   
> > > >           .section .text, "ax", %progbits
> > > 
> > > Since you call a C function, the code to save/restore a0/a1 needs
> > > to
> > > move here (from patch 4).
> > Thanks. It makes sense.
> > It would be better to move save/restore a0/a1 ( from patch 4 )code
> > here.
> > 
> > The only one reason I didn't do that before that calc_phys_offset
> > doesn't touch that and it is guaranteed that it will not ( as it
> > doesn't have arguments )
> 
> IIUC, the calling convention requires a0/a1 to be caller saved. So
> even 
> if they are not used for arguments, such callee is still free to use 
> them for internal purpose.
You are right.

I haven't seen that compiler use them if 'void' is passed to function
as an argument. But I agree that we have to follow the calling
convention to be sure that all is fine.

~ Oleksii