Re: [Xen-devel] [PATCH v8 4/6] remove late (on-demand) construction of IOMMU page tables

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 05 September 2019 15:30
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Julien Grall <julien.grall@xxxxxxx>; 
> Alexandru Isaila
> <aisaila@xxxxxxxxxxxxxxx>; PetrePircalabu <ppircalabu@xxxxxxxxxxxxxxx>; 
> Razvan Cojocaru
> <rcojocaru@xxxxxxxxxxxxxxx>; Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Roger 
> Pau Monne
> <roger.pau@xxxxxxxxxx>; Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>; 
> George Dunlap
> <George.Dunlap@xxxxxxxxxx>; Ian Jackson <Ian.Jackson@xxxxxxxxxx>; Stefano 
> Stabellini
> <sstabellini@xxxxxxxxxx>; Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; 
> Tamas K Lengyel
> <tamas@xxxxxxxxxxxxx>; Tim (Xen.org) <tim@xxxxxxx>; Wei Liu <wl@xxxxxxx>
> Subject: Re: [PATCH v8 4/6] remove late (on-demand) construction of IOMMU 
> page tables
> 
> On 02.09.2019 16:50, Paul Durrant wrote:
> > Now that there is a per-domain IOMMU-enable flag, which should be set if
> > any device is going to be passed through, stop deferring page table
> > construction until the assignment is done. Also don't tear down the tables
> > again when the last device is de-assigned; defer that task until domain
> > destruction.
> >
> > This allows the has_iommu_pt() helper and iommu_status enumeration to be
> > removed. Calls to has_iommu_pt() are simply replaced by calls to
> > is_iommu_enabled(). Remaining open-coded tests of iommu_hap_pt_share can
> > also be replaced by calls to iommu_use_hap_pt().
> > The arch_iommu_populate_page_table() and iommu_construct() functions become
> > redundant, as does the 'strict mode' dom0 page_list mapping code in
> > iommu_hwdom_init(), and iommu_teardown() can be made static is its only
> > remaining caller, iommu_domain_destroy(), is within the same source
> > module.
> >
> > All in all, about 220 lines of code are removed from the hypervisor.
> >
> > NOTE: This patch will cause a small amount of extra resource to be used
> >       to accommodate IOMMU page tables that may never be used, since the
> >       per-domain IOMMU-enable flag is currently set to the value of the
> >       global iommu_enable flag. A subsequent patch will add an option to
> >       the toolstack to allow it to be turned off if there is no intention
> >       to assign passthrough hardware to the domain.
> >       To account for the extra resource, 'iommu_memkb' has been added to
> >       domain_build_info. This patch sets it unconditionally to a value
> >       calculated based on the domain's maximum memory but, when the
> >       toolstack option mentioned above is added, it can be set to zero
> >       if the per-domain IOMMU-enable flag is turned off.
> >
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> 
> I've just stumbled across our earlier discussion on the still not
> sufficiently "x86/HVM: p2m_ram_ro is incompatible with device
> pass-through" of mine, and I wonder whether the implication from
> the change here is that people wanting p2m_ram_ro used on a guest
> would then need to force the IOMMU off for that guest (which aiui
> isn't possible until patch 6).

You wouldn't be able to force IOMMU off on a per-domain basis until patch #6 
but it could still be done globally.

  Paul

> 
> Jan
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