Re: [Xen-devel] [PATCH v2 0/2] AMD/IOMMU: re-work mode updating

["Durrant, Paul" <pdurrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of Jan
> Beulich
> Sent: 14 November 2019 16:42
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Juergen Gross <jgross@xxxxxxxx>; Sander Eikelenboom
> <linux@xxxxxxxxxxxxxx>; Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Subject: [Xen-devel] [PATCH v2 0/2] AMD/IOMMU: re-work mode updating
> 
> update_paging_mode() in the AMD IOMMU code expects to be invoked with
> the PCI devices lock held. The check occurring only when the mode
> actually needs updating, the violation of this rule by the majority
> of callers did go unnoticed until per-domain IOMMU setup was changed
> to do away with on-demand creation of IOMMU page tables.

Wouldn't it be safer to just get rid of update_paging_mode() and start with a 
reasonable number of levels?

  Paul

> 
> Unfortunately the only half way reasonable fix to this that I could
> come up with requires more re-work than would seem desirable at this
> time of the release process, but addressing the issue seems
> unavoidable to me as its manifestation is a regression from the
> IOMMU page table setup re-work. The change also isn't without risk
> of further regressions - if in patch 2 I've missed a code path that
> would also need to invoke the new hook, then this might mean non-
> working guests (with passed-through devices on AMD hardware).
> 
> 1: introduce GFN notification for translated domains
> 2: AMD/IOMMU: use notify_dfn() hook to update paging mode
> 
> Jan
> 
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