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SubjectRe: [PATCH v11 1/5] xen: move xen_setup_runstate_info and get_runstate_snapshot to drivers/xen/time.c
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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