Re: [Xen-devel] [PATCH v11 1/5] xen: move xen_setup_runstate_info and get_runstate_snapshot to drivers/xen/time.c

[Mark Rutland <mark.rutland@xxxxxxx>]

Hi,

> +static u64 get64(const u64 *p)
> +{
> +     u64 ret;
> +
> +     if (BITS_PER_LONG < 64) {
> +             u32 *p32 = (u32 *)p;
> +             u32 h, l;
> +
> +             /*
> +              * Read high then low, and then make sure high is
> +              * still the same; this will only loop if low wraps
> +              * and carries into high.
> +              * XXX some clean way to make this endian-proof?
> +              */
> +             do {
> +                     h = p32[1];
> +                     barrier();
> +                     l = p32[0];
> +                     barrier();
> +             } while (p32[1] != h);

I realise this is simply a move of existing code, but it may be better
to instead have:

do {
        h = READ_ONCE(p32[1]);
        l = READ_ONCE(p32[0]);
} while (READ_ONCE(p32[1] != h);

Which ensures that each load is a single access (though it almost
certainly would be anyway), and prevents the compiler from having to
reload any other memory locations (which the current barrier() usage
forces).

> +
> +             ret = (((u64)h) << 32) | l;
> +     } else
> +             ret = *p;

Likewise, this would be better as READ_ONCE(*p), to force a single
access.

> +
> +     return ret;
> +}

> +     do {
> +             state_time = get64(&state->state_entry_time);
> +             barrier();
> +             *res = *state;
> +             barrier();

You can also have:

        *res = READ_ONCE(*state);

That will which will handle the barriers implicitly.

Thanks,
Mark.

> +     } while (get64(&state->state_entry_time) != state_time);
> +}

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