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Date: Thu, 11 Apr 2013 11:49:52 +0200
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20130411094952.GB9165@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/5] memory: add ref/unref interface for
	MemroyRegionOps
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To: Liu Ping Fan <qemulist@gmail.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Anthony Liguori <aliguori@us.ibm.com>, Jan Kiszka <jan.kiszka@siemens.com>, Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel@nongnu.org, Vasilis Liaskovitis <vasilis.liaskovitis@profitbricks.com>, Paolo Bonzini <pbonzini@redhat.com>

On Mon, Apr 01, 2013 at 04:20:30PM +0800, Liu Ping Fan wrote:
> From: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
> 
> This pair of interface are optinal, except for those device which is
> used outside the biglock's protection for hot unplug.

Not sure if this comment is true.  Memory unplug safety is not about the
big lock, it's about whether a reference to memory is held *across* a
hot unplug operation.

So even code that is under the big lock can use a guest RAM buffer
across the event loop, and therefore be exposed to a RAM unplug!

Therefore inc/dec must be used if guest RAM is held across event loop
handler calls.  If the guest RAM access happens completely inside a
handler function, then it is not affected by hot plug and doesn't need
to do inc/dec.