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

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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.




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