Re: [PATCH v5 2/7] xen/riscv: set up fixmap mappings

[oleksii.kurochko@xxxxxxxxx]

On Tue, 2024-08-27 at 12:29 +0200, Jan Beulich wrote:
> > @@ -81,6 +82,18 @@ static inline void flush_page_to_ram(unsigned
> > long mfn, bool sync_icache)
> >       BUG_ON("unimplemented");
> >   }
> >   
> > +/* Write a pagetable entry. */
> > +static inline void write_pte(pte_t *p, pte_t pte)
> > +{
> > +    write_atomic(p, pte);
> > +}
> > +
> > +/* Read a pagetable entry. */
> > +static inline pte_t read_pte(pte_t *p)
> > +{
> > +    return read_atomic(p);
> 
> This only works because of the strange type trickery you're playing
> in
> read_atomic(). Look at x86 code - there's a strict expectation that
> the
> type can be converted to/from unsigned long. And page table accessors
> are written with that taken into consideration. Same goes for
> write_pte()
> of course, with the respective comment on the earlier patch in mind.
> 
> Otoh I see that Arm does something very similar. If you have a strong
> need / desire to follow that, then please at least split the two
> entirely separate aspects that patch 1 presently changes both in one
> go.
I am not 100% sure that type trick could be dropped easily for RISC-V:
1. I still need the separate C function for proper #ifdef-ing:
   #ifndef CONFIG_RISCV_32
       case 8: *(uint32_t *)res = readq_cpu(p); break;
   #endif
   
2. Because of the point 1 the change should be as following:
   -#define read_atomic(p) ({                                   \
   -    union { typeof(*(p)) val; char c[sizeof(*(p))]; } x_;   \
   -    read_atomic_size(p, x_.c, sizeof(*(p)));                \
   -    x_.val;                                                 \
   +#define read_atomic(p) ({                                 \
   +    unsigned long x_;                                     \
   +    read_atomic_size(p, &x_, sizeof(*(p)));               \
   +    (typeof(*(p)))x_;                                     \
    })
   But after that I think it will be an error: "conversion to non-scalar
   type requested" in the last line as *p points to pte_t.
   
   and we can't just in read_pte() change to:
   static inline pte_t read_pte(pte_t *p)
   {
       return read_atomic(&p->pte);
   }
   As in this cases it started it will return unsigned long but function
   expects pte_t. As an option read_pte() can be updated to:
   /* Read a pagetable entry. */
   static inline pte_t read_pte(pte_t *p)
   {
       return (pte_t) { .pte = read_atomic(&p->pte) };
   }
   
   But I am not sure that it is better then just have a union trick inside
   read_atomic() and then just have read_atomic(p) for read_pte().
   
   Am I missing something?
   
~ Oleksii