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

[Xiao Guangrong]

Ping...

Paolo, any comment?



On 12/02/2015 01:00 AM, Xiao Guangrong wrote:
> On 12/01/2015 06:17 PM, Paolo Bonzini wrote:
> > 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?
>
> Userfaultfd is excellent and has the ability to notify write event indeed,
> however, it is not suitable for the use case of shadow page.
>
> For the performance, shadow GPU is performance critical and requires
> frequently being switched, it is not good to handle it in userspace. And
> windows guest has many GPU tables and updates it frequently, that means,
> we need to write protect huge number of pages which are single page based,
> I am afraid userfaultfd can not handle this case efficiently.
>
> For the functionality, userfaultfd can not fill the need of shadow page
> because:
> - the page is keeping readonly, userfaultfd can not fix the fault and let
>    the vcpu progress (write access causes writeable gup).
>
> - the access need to be emulated, however, userfaultfd/kernel does not have
>    the ability to emulate the access as the access is trigged by guest, the
>    instruction info is stored in VMCS so that only KVM can emulate it.
>
> - shadow page needs to be notified after the emulation is finished as it
>    should know the new data written to the page to update its page hierarchy.
>    (some hardwares lack the 'retry' ability so the shadow page table need to
>     reflect the table in guest at any time).
>
> > 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.
>
> Yes. Very glad to see you like it. :)
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