linux-kernel - Re: [PATCH v4 2/2] xen/m2p: use GNTTABOP_unmap_and_replace to reinstate the original mapping

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Message-ID: <alpine.DEB.2.02.1308131638460.4893@kaball.uk.xensource.com>
Date:	Tue, 13 Aug 2013 16:38:51 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	<xen-devel@...ts.xensource.com>, <linux-kernel@...r.kernel.org>,
	<dcrisan@...xiant.com>, <alex@...x.org.uk>,
	<ian.campbell@...rix.com>
Subject: Re: [PATCH v4 2/2] xen/m2p: use GNTTABOP_unmap_and_replace to
 reinstate the original mapping

On Tue, 13 Aug 2013, Konrad Rzeszutek Wilk wrote:
> On Tue, Aug 13, 2013 at 12:17:18PM +0100, Stefano Stabellini wrote:
> > On Fri, 9 Aug 2013, Konrad Rzeszutek Wilk wrote:
> > > On Sun, Aug 04, 2013 at 03:39:41PM +0100, Stefano Stabellini wrote:
> > > > GNTTABOP_unmap_grant_ref unmaps a grant and replaces it with a 0
> > > > mapping instead of reinstating the original mapping.
> > > > Doing so separately would be racy.
> > > > 
> > > > To unmap a grant and reinstate the original mapping atomically we use
> > > > GNTTABOP_unmap_and_replace.
> > > > GNTTABOP_unmap_and_replace doesn't work with GNTMAP_contains_pte, so
> > > > don't use it for kmaps.  GNTTABOP_unmap_and_replace zeroes the mapping
> > > > passed in new_addr so we have to reinstate it, however that is a
> > > > per-cpu mapping only used for balloon scratch pages, so we can be sure that
> > > > it's not going to be accessed while the mapping is not valid.
> > > 
> > > This looks to be depend on a new structure, which is not in Linux kernel?
> > > Are you missing a dependency patch?
> > 
> > Nope, GNTTABOP_unmap_and_replace and struct gnttab_unmap_and_replace are
> > already present in include/xen/interface/grant_table.h.
> > 
> > 
> > > Shouldn't we use some logic to figure out which hypercall to use if the
> > > hypervisor does not support it?
> > 
> > GNTTABOP_unmap_and_replace is not a new hypercall, it has been supported
> > by Xen for a very long time.
> > 
> > In a previous iteration of this patch series, I did introduce a new
> > hypercall, but then I dropped it because I figured out that I could
> > achieve the same thing with the existing hypercall.
> 
> OK, Please tack on:
> 
> Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> 
> P.S.
> If you could stick it on devel/for-linus-3.12 that would be super. Thanks!

done!
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