Re: [PATCH v7 08/14] IOMMU/x86: prefill newly allocate page tables

["Durrant, Paul" <xadimgnik@xxxxxxxxx>]

On 05/07/2022 13:47, Jan Beulich wrote:
> Page tables are used for two purposes after allocation: They either
> start out all empty, or they are filled to replace a superpage.
> Subsequently, to replace all empty or fully contiguous page tables,
> contiguous sub-regions will be recorded within individual page tables.
> Install the initial set of markers immediately after allocation. Make
> sure to retain these markers when further populating a page table in
> preparation for it to replace a superpage.
>
> The markers are simply 4-bit fields holding the order value of
> contiguous entries. To demonstrate this, if a page table had just 16
> entries, this would be the initial (fully contiguous) set of markers:
>
> index  0 1 2 3 4 5 6 7 8 9 A B C D E F
> marker 4 0 1 0 2 0 1 0 3 0 1 0 2 0 1 0
>
> "Contiguous" here means not only present entries with successively
> increasing MFNs, each one suitably aligned for its slot, and identical
> attributes, but also a respective number of all non-present (zero except
> for the markers) entries.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Paul Durrant <paul@xxxxxxx>