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Re: [PATCH v2 15/18] IOMMU/x86: prefill newly allocate page tables
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Mon, 13 Dec 2021 16:51:31 +0100
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Paul Durrant <paul@xxxxxxx>, Kevin Tian <kevin.tian@xxxxxxxxx>
- Delivery-date: Mon, 13 Dec 2021 15:52:13 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Fri, Sep 24, 2021 at 11:54:58AM +0200, Jan Beulich wrote:
> Page table are used for two purposes after allocation: They either start
> out all empty, or they get 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, but also a
> respective number of all non-present entries.
I'm afraid I'm slightly lost with all this, please bear with me. Is
this just a performance improvement when doing super-page
replacements, or there's more to it?
Thanks, Roger.
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