Re: [Xen-devel] [PATCH v4 01/15] re-work commit 3e06b989 "IOMMU: make page table population preemptible"...

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Roger Pau Monne
> Sent: 01 August 2018 17:15
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>; George Dunlap
> <George.Dunlap@xxxxxxxxxx>; Andrew Cooper
> <Andrew.Cooper3@xxxxxxxxxx>; Ian Jackson <Ian.Jackson@xxxxxxxxxx>; Tim
> (Xen.org) <tim@xxxxxxx>; Julien Grall <julien.grall@xxxxxxx>; Jan Beulich
> <jbeulich@xxxxxxxx>
> Subject: Re: [Xen-devel] [PATCH v4 01/15] re-work commit 3e06b989
> "IOMMU: make page table population preemptible"...
> 
> On Wed, Aug 01, 2018 at 02:40:14PM +0100, Paul Durrant wrote:
> > ...to simplify the implementation and turn need_iommu back into a
> boolean.
> >
> > As noted in [1] the tri-state nature of need_iommu after commit 3e06b989
> is
> > confusing, as is the implementation of pre-emption using relmem_list.
> >
> > This patch instead uses a simple count of pages already populated stored in
> > the x86 variant of struct arch_iommu and skips over that number of pages
> > if arch_iommu_populate_page_table() is re-invoked after pre-emption.
> 
> Urg, all this is quite ugly. It would be quite better if the need for
> an IOMMU was decided before populating the p2m (ie: at domain_create
> time by passing an extra XEN_DOMCTL_CDF_iommu or some such), so that
> the normal addition of RAM would also populate the IOMMU page tables
> if not shared.

Given the problems that Jan has highlighted with using a skip count and 
essentially being forced to mess with the domain's page list, I think your 
suggestion is best. Then we can entirely bin arch_iommu_populate_page_table(). 
That is a bigger piece of work though so in the interest of no blocking the 
rest of this series I'll drop this patch and replace it with a simpler patch 
that's just sufficient to make need_iommu boolean again.

  Paul

> 
> > [1] https://lists.xenproject.org/archives/html/xen-devel/2018-
> 07/msg01870.html
> >
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> 
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> Thanks, Roger.

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