On Wed, Oct 02, 2019 at 10:10:18PM +0200, Paolo Bonzini wrote:
> On 02/10/19 19:04, Jerome Glisse wrote:
> > On Wed, Oct 02, 2019 at 06:18:06PM +0200, Paolo Bonzini wrote:
> >>>> 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)?
> >>
> >> The KVM inspector process is also (or can be) a QEMU that will have to
> >> create its own KVM guest page table. So if a page in the source VMA is
> >> unmapped we want:
> >>
> >> - the source KVM to invalidate its guest page table (done by the KVM MMU
> >> notifier)
> >>
> >> - the target VMA to be invalidated (easy using mirroring)
> >>
> >> - the target KVM to invalidate its guest page table, as a result of
> >> invalidation of the target VMA
> >
> > You can do the target KVM invalidation inside the mirroring invalidation
> > code.
>
> Why should the source and target KVMs behave differently? If the source
> invalidates its guest page table via MMU notifiers, so should the target.
>
> The KVM MMU notifier exists so that nothing (including mirroring) needs
> to know that there is KVM on the other side. Any interaction between
> KVM page tables and VMAs must be mediated by MMU notifiers, anything
> else is unacceptable.
>
> If it is possible to invoke the MMU notifiers around the calls to
> insert_pfn, that of course would be perfect.
Ok and yes you can do that exactly ie inside the mmu notifier callback
from the target. For instance it is as easy as:
target_mirror_notifier_start_callback(start, end) {
struct kvm_mirror_struct *kvmms = from_mmun(...);
unsigned long target_foff, size;
size = end - start;
target_foff = kvmms_convert_mirror_address(start);
take_lock(kvmms->mirror_fault_exclusion_lock);
unmap_mapping_range(kvmms->address_space, target_foff, size, 1);
drop_lock(kvmms->mirror_fault_exclusion_lock);
}
All that is needed is to make sure that vm_normal_page() will see those
pte (inside the process that is mirroring the other process) as special
which is the case either because insert_pfn() mark the pte as special or
the kvm device driver which control the vm_operation struct set a
find_special_page() callback that always return NULL, or the vma has
either VM_PFNMAP or VM_MIXEDMAP set (which is the case with insert_pfn).
So you can keep the existing kvm code unmodified.
Cheers,
Jérôme
