Re: [PATCH v5 12/23] xen/riscv: introduce io.h

[Oleksii <oleksii.kurochko@xxxxxxxxx>]

On Fri, 2024-03-08 at 08:26 +0100, Jan Beulich wrote:
> On 07.03.2024 21:54, Oleksii wrote:
> > On Thu, 2024-03-07 at 21:49 +0100, Oleksii wrote:
> > > On Thu, 2024-03-07 at 18:14 +0100, Jan Beulich wrote:
> > > > For plain writes it should at least be "=Qo" then, yes.
> > > Constraints Q is a machine specific constraint, and I am not sure
> > > that
> > > it makes sense to use "=o" only and probably it is a reason why
> > > it is
> > > enough only "r". Does it make sense?
> > Probably for RISC-V can be used:
> > RISC-V—config/riscv/constraints.md
> >    ...
> >    A
> >        An address that is held in a general-purpose register.
> >    ...
> 
> Just from the description I would have said no, but looking at what
> "A"
> actually expands to it is indeed RISC-V's counterpart of Arm's "Q".
> So
> yes, this looks like what amo* want to use, and then as a real
> operand,
> not just a fake one.
I am not sure that I know how to check correctly how "A" expands, but I
tried to look at code which will be generated with and without
constraints and it is the same:
   // static inline void __raw_writel(uint32_t val, volatile void
   __iomem *addr)
   // {
   //     asm volatile ( "sw %0, 0(%1)" : : "r" (val), "r"(addr) );
   // }
   
   static inline void __raw_writel(uint32_t val, volatile void __iomem
   *addr)
   {
       asm volatile ( "sw %0, %1" : : "r" (val), "Ao" (*(volatile
   uint32_t __force *)addr) );
   }
   
   ffffffffc003d774:       aabbd7b7                lui     a5,0xaabbd
   ffffffffc003d778:       cdd78793                add     a5,a5,-803 #
   ffffffffaabbccdd <start-0x15443323>
   ffffffffc003d77c:       f8f42423                sw      a5,-120(s0)
   ffffffffc003d780:       0140000f                fence   w,o
   

> 
> > AArch64 family—config/aarch64/constraints.md:
> >    ...
> >    Q
> >        A memory address which uses a single base register with no
> >    offset
> >    ...
> > Also 'no offset' explains why it was added 'o' constraint for Arm
> > additionally.
> 
> I don't think it does.
> 
> Jan