linux-kernel - Re: [PATCH] xen: modify kernel mappings corresponding to granted pages

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Date:	Tue, 6 Sep 2011 17:19:01 +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" <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <Jeremy.Fitzhardinge@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ian Campbell <Ian.Campbell@...citrix.com>
Subject: Re: [PATCH] xen: modify kernel mappings corresponding to granted
 pages

On Tue, 9 Aug 2011, Konrad Rzeszutek Wilk wrote:
> On Tue, Jul 26, 2011 at 05:55:45PM +0100, stefano.stabellini@...citrix.com wrote:
> > From: Stefano Stabellini <stefano.stabellini@...citrix.com>
> >
> > If we want to use granted pages for AIO, changing the mappings of a user
> > vma and the corresponding p2m is not enough, we also need to update the
> > kernel mappings accordingly.
> > On x86_64 it is easy, we can issue another HYPERVISOR_grant_table_op
> > right away in m2p_add_override. We can remove the mappings using another
> > HYPERVISOR_grant_table_op in m2p_remove_override.
> > On x86_32 it is more difficult because the pages are highmem pages and
> > therefore we need to catch the set_pte that tries to map a granted page
> > and issue an HYPERVISOR_grant_table_op instead.
> > Same thing for unmapping them: we need to catch the pte clear or the
> > set_pte that try to unmap a granted page and issue an
> > HYPERVISOR_grant_table_op.
> 
> So I hadn't looked at this in detail, but I wonder if we can use the
> MULTIcall for this? It looks like we need to do two hypercalls so why
> not batch it?

It wasn't easy but I managed to use multicalls in both m2p override and
the highmem counterparts in xen/mmu.c.
I'll send the multicall changes as a separate patch because I think
is going to be easier to read.
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