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Re: [Xen-devel] RFC: [0/2] Remove netloop by lazy copying in netback

To: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] RFC: [0/2] Remove netloop by lazy copying in netback
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Mon, 26 Mar 2007 19:36:16 +0100
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On 26/3/07 03:19, "Herbert Xu" <herbert@xxxxxxxxxxxxxxxxxxx> wrote:

> We need something that can replace a p2m entry atomically and more
> importantly swap two p2m entries rather than setting one and unmapping
> the other.  The former is because we can't easily process p2m page
> faults in the guest.  The latter is because we stlil need to unmap the
> grant table entry after this operation so we have to keep the entry
> around.

Can't we wrap the 'swap around' critical section in an irq-safe spinlock?
All we'd need to do from the page-fault handler then is a barrier on that
spinlock (i.e, wait for it to be released). Netback can simply copy the page
to new memory frame, unmap the grant, then relocate the new memory frame's
pseudophysical address.

 -- Keir

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