Re: [PATCH 1/2] KVM: MMU: Do not treat ZONE_DEVICE pages as being reserved

[Dan Williams]

On Wed, Nov 6, 2019 at 3:39 PM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Wed, Nov 06, 2019 at 03:20:11PM -0800, Dan Williams wrote:
> > After some more thought I'd feel more comfortable just collapsing the
> > ZONE_DEVICE case into the VM_IO/VM_PFNMAP case. I.e. with something
> > like this (untested) that just drops the reference immediately and let
> > kvm_is_reserved_pfn() do the right thing going forward.
>
> This will break the page fault flow, as it will allow the page to be
> whacked before KVM can ensure it will get proper notification from the
> mmu_notifier.  E.g. KVM would install the PFN in its secondary MMU after
> getting the invalidate notification for the PFN.

How do mmu notifiers get held off by page references and does that
machinery work with ZONE_DEVICE? Why is this not a concern for the
VM_IO and VM_PFNMAP case?