Re: [PATCH v7 00/14] IOMMU: superpage support when not sharing pagetables

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.07.2022 14:41, Jan Beulich wrote:
> For a long time we've been rather inefficient with IOMMU page table
> management when not sharing page tables, i.e. in particular for PV (and
> further specifically also for PV Dom0) and AMD (where nowadays we never
> share page tables). While up to about 3.5 years ago AMD code had logic
> to un-shatter page mappings, that logic was ripped out for being buggy
> (XSA-275 plus follow-on).
> 
> This series enables use of large pages in AMD and Intel (VT-d) code;
> Arm is presently not in need of any enabling as pagetables are always
> shared there. It also augments PV Dom0 creation with suitable explicit
> IOMMU mapping calls to facilitate use of large pages there. Depending
> on the amount of memory handed to Dom0 this improves booting time
> (latency until Dom0 actually starts) quite a bit; subsequent shattering
> of some of the large pages may of course consume some of the saved time.
> 
> Known fallout has been spelled out here:
> https://lists.xen.org/archives/html/xen-devel/2021-08/msg00781.html
> 
> See individual patches for details on the v7 changes.
> 
> 01: iommu: add preemption support to iommu_{un,}map()
> 02: IOMMU/x86: perform PV Dom0 mappings in batches

Paul,

without meaning this to be a ping, may I ask whether - with Roger away
for the next two months - you could find time to review these first two
patches? I think this would then allow the entire series to go in.

Thanks, Jan

> 03: IOMMU/x86: support freeing of pagetables
> 02: IOMMU/x86: new command line option to suppress use of superpage mappings
> 03: AMD/IOMMU: allow use of superpage mappings
> 04: VT-d: allow use of superpage mappings
> 05: x86: introduce helper for recording degree of contiguity in page tables
> 06: IOMMU/x86: prefill newly allocate page tables
> 07: AMD/IOMMU: free all-empty page tables
> 08: VT-d: free all-empty page tables
> 09: AMD/IOMMU: replace all-contiguous page tables by superpage mappings
> 10: VT-d: replace all-contiguous page tables by superpage mappings
> 11: IOMMU/x86: add perf counters for page table splitting / coalescing
> 12: VT-d: fold dma_pte_clear_one() into its only caller
> 
> While not directly related (except that making this mode work properly
> here was a fair part of the overall work), at this occasion I'd also
> like to renew my proposal to make "iommu=dom0-strict" the default going
> forward. It already is not only the default, but the only possible mode
> for PVH Dom0.
> 
> Jan
>