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

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Date:	Thu, 15 Feb 2007 22:58:01 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Andi Kleen <ak@....de>, linux-kernel@...r.kernel.org,
	virtualization@...ts.osdl.org, xen-devel@...ts.xensource.com,
	Chris Wright <chrisw@...s-sol.org>,
	Zachary Amsden <zach@...are.com>,
	Ian Pratt <ian.pratt@...source.com>,
	Christian Limpach <Christian.Limpach@...cam.ac.uk>
Subject: Re: [patch 18/21] Xen-paravirt: Add Xen grant table support

Andrew Morton wrote:
> On Thu, 15 Feb 2007 18:25:07 -0800 Jeremy Fitzhardinge <jeremy@...p.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
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