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SubjectRe: [PATCH 00/11] KVM: x86: track guest page access
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On 30/11/2015 19:26, Xiao Guangrong wrote:
> This patchset introduces the feature which allows us to track page
> access in guest. Currently, only write access tracking is implemented
> in this version.
>
> Four APIs are introduces:
> - kvm_page_track_add_page(kvm, gfn, mode), single guest page @gfn is
> added into the track pool of the guest instance represented by @kvm,
> @mode specifies which kind of access on the @gfn is tracked
>
> - kvm_page_track_remove_page(kvm, gfn, mode), is the opposed operation
> of kvm_page_track_add_page() which removes @gfn from the tracking pool.
> gfn is no tracked after its last user is gone
>
> - kvm_page_track_register_notifier(kvm, n), register a notifier so that
> the event triggered by page tracking will be received, at that time,
> the callback of n->track_write() will be called
>
> - kvm_page_track_unregister_notifier(kvm, n), does the opposed operation
> of kvm_page_track_register_notifier(), which unlinks the notifier and
> stops receiving the tracked event
>
> The first user of page track is non-leaf shadow page tables as they are
> always write protected. It also gains performance improvement because
> page track speeds up page fault handler for the tracked pages. The
> performance result of kernel building is as followings:
>
> before after
> real 461.63 real 455.48
> user 4529.55 user 4557.88
> sys 1995.39 sys 1922.57

For KVM-GT, as far as I know Andrea Arcangeli is working on extending
userfaultfd to tracking write faults only. Perhaps KVM-GT can do
something similar, where KVM gets the write tracking functionality for
free through the MMU notifiers. Any thoughts on this?

Applying your technique to non-leaf shadow pages actually makes this
series quite interesting. :) Shadow paging is still in use for nested
EPT, so it's always a good idea to speed it up.

Paolo


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