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

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Message-Id: <20070215225229.6b63d050.akpm@linux-foundation.org>
Date:	Thu, 15 Feb 2007 22:52:29 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeremy Fitzhardinge <jeremy@...p.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

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?
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