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

[Jan Beulich <jbeulich@xxxxxxxx>]

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).

Jan

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