On Wed, Oct 02, 2019 at 03:46:30PM +0200, Paolo Bonzini wrote:
> On 02/10/19 21:27, Jerome Glisse wrote:
> > On Tue, Sep 10, 2019 at 07:49:51AM +0000, Mircea CIRJALIU - MELIU wrote:
> >>> On 05/09/19 20:09, Jerome Glisse wrote:
> >>>> Not sure i understand, you are saying that the solution i outline
> >>>> above does not work ? If so then i think you are wrong, in the above
> >>>> solution the importing process mmap a device file and the resulting
> >>>> vma is then populated using insert_pfn() and constantly keep
> >>>> synchronize with the target process through mirroring which means that
> >>>> you never have to look at the struct page ... you can mirror any kind
> >>>> of memory from the remote process.
> >>>
> >>> If insert_pfn in turn calls MMU notifiers for the target VMA (which would be
> >>> the KVM MMU notifier), then that would work. Though I guess it would be
> >>> possible to call MMU notifier update callbacks around the call to insert_pfn.
> >>
> >> Can't do that.
> >> First, insert_pfn() uses set_pte_at() which won't trigger the MMU notifier on
> >> the target VMA. It's also static, so I'll have to access it thru vmf_insert_pfn()
> >> or vmf_insert_mixed().
> >
> > Why would you need to target mmu notifier on target vma ?
>
> If the mapping of the source VMA changes, mirroring can update the
> target VMA via insert_pfn. But what ensures that KVM's MMU notifier
> dismantles its own existing page tables (so that they can be recreated
> with the new mapping from the source VMA)?
>
So just to make sure i follow we have:
- qemu process on host with anonymous vma
-> host cpu page table
- kvm which maps host anonymous vma to guest
-> kvm guest page table
- kvm inspector process which mirror vma from qemu process
-> inspector process page table
AFAIK the KVM notifier's will clear the kvm guest page table whenever
necessary (through kvm_mmu_notifier_invalidate_range_start). This is
what ensure that KVM's dismatles its own mapping, it abides to mmu-
notifier callbacks. If you did not you would have bugs (at least i
expect so). Am i wrong here ?
The mirroring kernel driver would also register the notifier against
the quemu process and would also abide to notifier callbacks.
What you want to maintain at all times is that none of the actors
above ever look at different page for the same virtual address (ie
one looking at older page while another look at new page).
This is where you have helper like HMM that make sure that you can
not populate the mirroring vma while a notifier is on going. Which
means that everything is serialize on the notifier.
Cheers,
Jérôme
