Re: [Xen-devel] [PATCH] libxc: try to find last used pfn when migrating

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 27/11/15 14:50, Juergen Gross wrote:
> For migration the last used pfn of a guest is needed to size the
> logdirty bitmap and as an upper bound of the page loop. Unfortunately
> there are pv-kernels advertising a much higher maximum pfn as they
> are really using in order to support memory hotplug. This will lead
> to allocation of much more memory in Xen tools during migration as
> really needed.
> 
> Try to find the last used guest pfn of a pv-domu by scanning the p2m
> tree from the last entry towards it's start and search for an entry
> not being invalid.
> 
> Normally the mid pages of the p2m tree containing all invalid entries
> are being reused, so we can just scan the top page for identical
> entries and skip them but the first one.

This has been fixed in Linux by "x86/xen/p2m: hint at the last populated
P2M entry" 98dd166ea3a3c3b57919e20d9b0d1237fcd0349d which is tagged for
stable.

Do we really need a toolstack fix as well?

David

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