On 30.08.2024 13:55, [email protected] wrote:
> 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.
Of course.
> 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) };
> }
That's what's needed.
> 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().
It's largely up to you. My main request is that things end up / remain
consistent. Which way round is secondary, and often merely a matter of
suitably justifying the choice made.
Jan
