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

[Oleksii <oleksii.kurochko@xxxxxxxxx>]

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.

> If the omission of respective constraints here
> and below is intentional, I think a comment (covering all instances)
> is
> needed. Note that while supposedly cloned from Arm code, Arm variants
> do
> have such constraints in Linux.
> 
It uses this constains only in arm32:
#define __raw_writeb __raw_writeb
static inline void __raw_writeb(u8 val, volatile void __iomem *addr)
{
        asm volatile("strb %1, %0"
                     : : "Qo" (*(volatile u8 __force *)addr), "r"
(val));
}

But in case of arm64:

#define __raw_writeb __raw_writeb
static __always_inline void __raw_writeb(u8 val, volatile void __iomem
*addr)
{
        asm volatile("strb %w0, [%1]" : : "rZ" (val), "r" (addr));
}

And again looking at the defintion they use different option of strb
instruction, and in case of strb they use [%1] which tells compiler
that %1 is addressed which should be dereferenced.

~ Oleksii