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Re: [Xen-devel] Re: blktap: Sync with XCP, dropping zero-copy.

On Mon, 2010-11-15 at 19:34 +0000, Jeremy Fitzhardinge wrote: 
> On 11/15/2010 11:19 AM, Ian Campbell wrote:
> > On Mon, 2010-11-15 at 18:27 +0000, Jeremy Fitzhardinge wrote:
> >> Then we'd have a set of frames whose lifetimes are being determined by
> >> some other subsystem.  We can either maintain a list of them and poll
> >> waiting for them to become free, or just release them and let them be
> >> managed by the normal kernel lifetime rules (which requires that the
> >> memory attached to them be completely normal, of course). 
> > GNTTABOP_unmap_and_replace is probably the answer to this? This is what
> > netback uses (via gnttab_copy_grant_page) when it wants to copy and skb
> > which has remained in flight for too long.
> >
> > Maybe this doesn't work since the memcpy and replace are not atomic and
> > we don't have a lock to use, since we don't know which subsystem holds
> > the reference. However it should work if we don't really care about the
> > contents of the replacement too much, which I don't think we do in this
> > case since the data could just as easily get clobbered by the userspace
> > process which thinks the write has completed fully, in which case we
> > just replace the grant mapping without bothering with the copy.
> Well, if atomicity is important (and it is a relatively rare event), I
> dare say Xen could de-schedule all the VCPUs a lot more efficiently than
> we could run stop_machine() from within the kernel...


GNTTABOP_unmap_and_replace is itself atomic according to the comment in
the header, it's not clear what it is atomic WRT, presumably just other
gnttab or mmu ops. In any case it doesn't handle synchronising the
content of the old and new pages so if we care about that we would need
a new GNTAPOP_copy_unmap_and_replace or something like that.


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