Re: [PATCH v4 00/21] IOMMU: superpage support when not sharing pagetables

[Jan Beulich <jbeulich@suse.com>]

On 25.04.2022 10:29, 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 2.5 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
> 
> There's a dependency on 'PCI: replace "secondary" flavors of
> PCI_{DEVFN,BDF,SBDF}()', in particular by patch 8. Its prereq patch
> still lacks an Arm ack, so it couldn't go in yet.
> 
> I'm inclined to say "of course" there are also a few seemingly unrelated
> changes included here, which I just came to consider necessary or at
> least desirable (in part for having been in need of adjustment for a
> long time) along the way. Some of these changes are likely independent
> of the bulk of the work here, and hence may be fine to go in ahead of
> earlier patches.
> 
> See individual patches for details on the v4 changes.
> 
> 01: AMD/IOMMU: correct potentially-UB shifts
> 02: IOMMU: simplify unmap-on-error in iommu_map()
> 03: IOMMU: add order parameter to ->{,un}map_page() hooks
> 04: IOMMU: have iommu_{,un}map() split requests into largest possible chunks

These first 4 patches are in principle ready to go in. If only there
wasn't (sadly once again) the unclear state with comments on the
first 2 that you had given on Apr 27. I did reply verbally, and hence
I'm intending to commit these 4 by the end of the week - on the
assumption that no response to my replies means I've sufficiently
addressed the concerns - unless I hear back otherwise.

Thanks for you understanding, Jan