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Re: [Qemu-devel] [PATCH 1/5] memory: add ref/unref interface for MemroyR

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH 1/5] memory: add ref/unref interface for MemroyRegionOps
Date: Thu, 11 Apr 2013 11:49:52 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Mon, Apr 01, 2013 at 04:20:30PM +0800, Liu Ping Fan wrote:
> From: Liu Ping Fan <address@hidden>
> 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.

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