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

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] RFC: [0/2] Remove netloop by lazy copying in netback
From: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Date: Tue, 27 Mar 2007 07:08:28 +1000
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On Mon, Mar 26, 2007 at 07:36:16PM +0100, Keir Fraser wrote:
> 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.

That works fine for the x86 case.  But when it's auto-translated,
you won't even get a page fault in the guest because the guest PTE
is unchanged and completely valid.

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