Re: [patch 18/21] Xen-paravirt: Add Xen grant table support

Jeremy Fitzhardinge
2007-02-16T06:58:04Z

Andrew Morton wrote:
> On Thu, 15 Feb 2007 18:25:07 -0800 Jeremy Fitzhardinge <jeremy@goop.org> wrote:
>
>
>> +int gnttab_grant_foreign_access(domid_t domid, unsigned long frame,
>> + int readonly)
>> +{
>> + int ref;
>> +
>> + if (unlikely((ref = get_free_entry()) == -1))
>> + return -ENOSPC;
>> +
>> + shared[ref].frame = frame;
>> + shared[ref].domid = domid;
>> + wmb();
>> + shared[ref].flags = GTF_permit_access | (readonly ? GTF_readonly : 0);
>> +
>> + return ref;
>> +}
>> +EXPORT_SYMBOL_GPL(gnttab_grant_foreign_access);
>>
>
> We have lots of open-coded mysteriously unexplained barriers in here.
>
> I assume they're not smp_wmb() because this could be a !SMP guest talking
> to an SMP host?
>

Yeah. The grant tables refer to pages which are shared with other
domains, so they could be running on other cpus even if this domain is
UP. There's a lockless protocol going on here, but I'll need to look it
up and sprinkle some comments.

J