Re: [PATCH v3 kvm/queue 11/16] KVM: Add kvm_map_gfn_range

[Chao Peng]

On Wed, Jan 05, 2022 at 05:03:23PM +0000, Sean Christopherson wrote:
> On Wed, Jan 05, 2022, Chao Peng wrote:
> > On Tue, Jan 04, 2022 at 05:31:30PM +0000, Sean Christopherson wrote:
> > > On Fri, Dec 31, 2021, Chao Peng wrote:
> > > > On Fri, Dec 24, 2021 at 12:13:51PM +0800, Chao Peng wrote:
> > > > > On Thu, Dec 23, 2021 at 06:06:19PM +0000, Sean Christopherson wrote:
> > > > > > On Thu, Dec 23, 2021, Chao Peng wrote:
> > > > > > > This new function establishes the mapping in KVM page tables for a
> > > > > > > given gfn range. It can be used in the memory fallocate callback 
> > > > > > > for
> > > > > > > memfd based memory to establish the mapping for KVM secondary MMU 
> > > > > > > when
> > > > > > > the pages are allocated in the memory backend.
> > > > > > 
> > > > > > NAK, under no circumstance should KVM install SPTEs in response to 
> > > > > > allocating
> > > > > > memory in a file.   The correct thing to do is to invalidate the 
> > > > > > gfn range
> > > > > > associated with the newly mapped range, i.e. wipe out any shared 
> > > > > > SPTEs associated
> > > > > > with the memslot.
> > > > > 
> > > > > Right, thanks.
> > > > 
> > > > BTW, I think the current fallocate() callback is just useless as long as
> > > > we don't want to install KVM SPTEs in response to allocating memory in a
> > > > file. The invalidation of the shared SPTEs should be notified through 
> > > > mmu_notifier of the shared memory backend, not memfd_notifier of the
> > > > private memory backend.
> > > 
> > > No, because the private fd is the final source of truth as to whether or 
> > > not a
> > > GPA is private, e.g. userspace may choose to not unmap the shared backing.
> > > KVM's rule per Paolo's/this proposoal is that a GPA is private if it has 
> > > a private
> > > memslot and is present in the private backing store.  And the other core 
> > > rule is
> > > that KVM must never map both the private and shared variants of a GPA 
> > > into the
> > > guest.
> > 
> > That's true, but I'm wondering if zapping the shared variant can be
> > handled at the time when the private one gets mapped in the KVM page
> > fault. No bothering the backing store to dedicate a callback to tell
> > KVM.
> 
> Hmm, I don't think that would work for the TDP MMU due to page faults taking
> mmu_lock for read.  E.g. if two vCPUs concurrently fault in both the shared 
> and
> private variants, a race could exist where the private page fault sees the gfn
> as private and the shared page fault sees it as shared.  In that case, both 
> faults
> will install a SPTE and KVM would end up running with both variants mapped 
> into the
> guest.
> 
> There's also a performance penalty, as KVM would need to walk the shared EPT 
> tree
> on every private page fault.

Make sense.

Thanks,
Chao