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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 10:35:31 +1000
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On Tue, Mar 27, 2007 at 01:33:58AM +0100, Keir Fraser wrote:
> >> 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.
> How about you invalidate the PTE for the duration of the critical section.
> It's a bit skanky, but would work around this issue quite nicely!

Maybe I am missing something.  Because I thought we've agreed that
we can't (or don't want to) do the PTE trick because of the large
pages (16MB at least) used for the identity mapping.

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