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

[Oleksii <oleksii.kurochko@xxxxxxxxx>]

On Thu, 2024-03-07 at 14:24 +0100, Jan Beulich wrote:
> On 07.03.2024 14:01, Oleksii wrote:
> > On Wed, 2024-03-06 at 15:13 +0100, Jan Beulich wrote:
> > > > +/* Generic IO read/write.  These perform native-endian
> > > > accesses.
> > > > */
> > > > +static inline void __raw_writeb(uint8_t val, volatile void
> > > > __iomem
> > > > *addr)
> > > > +{
> > > > +    asm volatile ( "sb %0, 0(%1)" : : "r" (val), "r" (addr) );
> > > > +}
> > > 
> > > I realize this is like Linux has it, but how is the compiler to
> > > know
> > > that
> > > *addr is being access here? 
> > Assembler syntax told compiler that. 0(%1) - means that the memory
> > location pointed to by the address in register %1.
> 
> No, the compiler doesn't decompose the string to figure how operands
> are used. That's what the constraints are for. The only two things
> the
> compiler does with the string is replace % operators and count line
> separators.
It looks like I am missing something.

addr -> a some register ( because of "r" contraint ).
val -> is also register ( because of "r" contraint ).

So the compiler will update instert an instruction:
 sb reg1, 0(reg2)

what means *(uint_8 *)reg2 = (uint8_t)reg1.

What am I missing?

~ Oleksii