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

From: Paul Durrant <Paul.Durrant@citrix.com>
To: 'Jan Beulich' <JBeulich@suse.com>
Cc: 'Stefano Stabellini' <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	Andrew Cooper <Andrew.Cooper3@citrix.com>,
	"Tim (Xen.org)" <tim@xen.org>,
	George Dunlap <George.Dunlap@citrix.com>,
	'Julien Grall' <julien.grall@arm.com>,
	'xen-devel' <xen-devel@lists.xenproject.org>,
	Ian Jackson <Ian.Jackson@citrix.com>
Subject: Re: [PATCH v4 01/15] re-work commit 3e06b989 "IOMMU: make page table population preemptible"...
Date: Thu, 2 Aug 2018 08:04:49 +0000	[thread overview]
Message-ID: <3d83229e473b4430969e1dbde9ef7125@AMSPEX02CL03.citrite.net> (raw)
In-Reply-To: <74e395deae9549c3a05fccb1241c35ae@AMSPEX02CL03.citrite.net>

> -----Original Message-----
> From: Paul Durrant
> Sent: 02 August 2018 09:03
> To: 'Jan Beulich' <JBeulich@suse.com>
> Cc: Julien Grall <julien.grall@arm.com>; Andrew Cooper
> <Andrew.Cooper3@citrix.com>; Wei Liu <wei.liu2@citrix.com>; George
> Dunlap <George.Dunlap@citrix.com>; Ian Jackson <Ian.Jackson@citrix.com>;
> Stefano Stabellini <sstabellini@kernel.org>; xen-devel <xen-
> devel@lists.xenproject.org>; Konrad Rzeszutek Wilk
> <konrad.wilk@oracle.com>; Tim (Xen.org) <tim@xen.org>
> Subject: RE: [PATCH v4 01/15] re-work commit 3e06b989 "IOMMU: make
> page table population preemptible"...
> 
> > -----Original Message-----
> > From: Jan Beulich [mailto:JBeulich@suse.com]
> > Sent: 02 August 2018 08:20
> > To: Paul Durrant <Paul.Durrant@citrix.com>
> > Cc: Julien Grall <julien.grall@arm.com>; Andrew Cooper
> > <Andrew.Cooper3@citrix.com>; Wei Liu <wei.liu2@citrix.com>; George
> > Dunlap <George.Dunlap@citrix.com>; Ian Jackson
> <Ian.Jackson@citrix.com>;
> > Stefano Stabellini <sstabellini@kernel.org>; xen-devel <xen-
> > devel@lists.xenproject.org>; Konrad Rzeszutek Wilk
> > <konrad.wilk@oracle.com>; Tim (Xen.org) <tim@xen.org>
> > Subject: Re: [PATCH v4 01/15] re-work commit 3e06b989 "IOMMU: make
> > page table population preemptible"...
> >
> > >>> On 01.08.18 at 15:40, <paul.durrant@citrix.com> 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.
> >
> > Well, yes, I would have used that model in said commit if it was reliable,
> > but it isn't: What if the list of pages changed between two (re-)invocations?
> 
> Is that really going to happen? This is the result of a domctl, which is a tools-
> only hypercall right?

Oh, I see what you mean... the guest could do something like a decrease_reservation... I was overlooking that setting up the iommu is happening while the guest is live. Would it be reasonable to domain_pause() for safety then?

  Paul

> 
> > Furthermore, with huge enough a guest even the skipping of the already
> > processed several million pages may exhaust the time acceptable between
> > preemption points.
> 
> Ok, I'll keep a pointer into the page list instead.
> 
>   Paul
> 
> >
> > Jan
> >

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