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

[Oleksii <oleksii.kurochko@xxxxxxxxx>]

On Fri, 2023-07-07 at 11:35 +0200, Jan Beulich wrote:
> On 07.07.2023 11:12, 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 )
> 
> How does a function not having parameters guarantee that registers
> used for parameter passing aren't touched? Inside a function, the
> compiler is free to use argument-passing registers just like other
> temporary ones; their values don't need preserving, from all I know
> (otherwise the RISC-V ABI would be different to all other ABIs I
> know of).
Well, you are right that it doesn't guarantee and the calling
convention tells that arg registers should be saved/restored
before/after function call.

But I haven't seen yet that compiler touch arg registers if function
accepts 'void' as an function argument. So 'guarantee' isn't correct
word.

Thanks for the note.

~ Oleksii