linux-kernel - Re: [PATCH 00/11] KVM: x86: track guest page access

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Date:	Tue, 1 Dec 2015 11:17:30 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
Cc:	gleb@...nel.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 00/11] KVM: x86: track guest page access



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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