From mboxrd@z Thu Jan  1 00:00:00 1970
From: Keir Fraser <keir.xen@gmail.com>
Subject: Re: [PATCH 3/4] VMX: Add posted interrupt supporting
Date: Tue, 09 Apr 2013 10:18:55 +0100
Message-ID: <CD899B8F.205A4%keir.xen@gmail.com>
References: <5163F4C502000078000CBA9D@nat28.tlf.novell.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Return-path: <xen-devel-bounces@lists.xen.org>
In-Reply-To: <5163F4C502000078000CBA9D@nat28.tlf.novell.com>
List-Unsubscribe: <http://lists.xen.org/cgi-bin/mailman/options/xen-devel>,
	<mailto:xen-devel-request@lists.xen.org?subject=unsubscribe>
List-Post: <mailto:xen-devel@lists.xen.org>
List-Help: <mailto:xen-devel-request@lists.xen.org?subject=help>
List-Subscribe: <http://lists.xen.org/cgi-bin/mailman/listinfo/xen-devel>,
	<mailto:xen-devel-request@lists.xen.org?subject=subscribe>
Sender: xen-devel-bounces@lists.xen.org
Errors-To: xen-devel-bounces@lists.xen.org
To: Jan Beulich <JBeulich@suse.com>, Yang Z Zhang <yang.z.zhang@intel.com>
Cc: Xiantao Zhang <xiantao.zhang@intel.com>, "xen-devel@lists.xen.org" <xen-devel@lists.xen.org>
List-Id: xen-devel@lists.xenproject.org

On 09/04/2013 10:00, "Jan Beulich" <JBeulich@suse.com> wrote:

>>>> Can't you use vlapic_set_vector() here (even if that means
>>>> looping over vectors individually rather than groups), to add the
>>>> necessary atomicity (I don't see how you avoid races with other
>>>> updates) and to avoid the ugly cast?
>> Which races?
> 
> vlapic.c also updates the APIC_IRR bit array, and hence (unless
> you can guarantee that now and forever such updates only
> happen when the subject vCPU is current, which I don't think is
> the case)

It's not the case. Which is one reason the IRR updates (and I think all
bitmap updates) in vlapic.c are atomic.

 -- Keir

> your non-atomic read-modify-write operation here can
> discard an update done in vlapic.c. And am I wrong in recalling
> that the CPU may actually also on its own update that bit array?