Re: [PATCH 06/25] xen: Core Xen implementation

Jeremy Fitzhardinge
2007-04-27T07:07:38Z

Andi Kleen wrote:
>> + /* convert from IF type flag */
>> + flags = !(flags & X86_EFLAGS_IF);
>> + vcpu = x86_read_percpu(xen_vcpu);
>> + vcpu->evtchn_upcall_mask = flags;
>> + if (flags == 0) {
>> + barrier(); /* unmask then check (avoid races) */
>>
>
> Don't you need a rmb() here then? The CPU could speculate reads
> (more occurrences)
>

Is rmb() sufficient? It will stop a speculative read on the pending
flag, but will it make sure the write has happened by then? Ie, is it a
write-vs-read barrier, or just a read-vs-read?
Documentation/memory-barriers.txt suggests not.

J