Re: [PATCH v2 20/27] KVM: x86/mmu: Use page-track notifiers iff there are external users

[Sean Christopherson <seanjc@google.com>]

On Wed, Mar 15, 2023, Sean Christopherson wrote:
> On Wed, Mar 15, 2023, Yan Zhao wrote:
> > On Fri, Mar 10, 2023 at 04:22:51PM -0800, Sean Christopherson wrote:
> > > Disable the page-track notifier code at compile time if there are no
> > > external users, i.e. if CONFIG_KVM_EXTERNAL_WRITE_TRACKING=n.  KVM itself
> > > now hooks emulated writes directly instead of relying on the page-track
> > > mechanism.
> > > 
> > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > > ---
> > >  arch/x86/include/asm/kvm_host.h       |  2 ++
> > >  arch/x86/include/asm/kvm_page_track.h |  2 ++
> > >  arch/x86/kvm/mmu/page_track.c         |  9 ++++-----
> > >  arch/x86/kvm/mmu/page_track.h         | 29 +++++++++++++++++++++++----
> > >  4 files changed, 33 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > > index 1a4225237564..a3423711e403 100644
> > > --- a/arch/x86/include/asm/kvm_host.h
> > > +++ b/arch/x86/include/asm/kvm_host.h
> > > @@ -1265,7 +1265,9 @@ struct kvm_arch {
> > >  	 * create an NX huge page (without hanging the guest).
> > >  	 */
> > >  	struct list_head possible_nx_huge_pages;
> > > +#ifdef CONFIG_KVM_EXTERNAL_WRITE_TRACKING
> > >  	struct kvm_page_track_notifier_head track_notifier_head;
> > > +#endif
> > >  	/*
> > >  	 * Protects marking pages unsync during page faults, as TDP MMU page
> > >  	 * faults only take mmu_lock for read.  For simplicity, the unsync
> > > diff --git a/arch/x86/include/asm/kvm_page_track.h b/arch/x86/include/asm/kvm_page_track.h
> > > index deece45936a5..53c2adb25a07 100644
> > > --- a/arch/x86/include/asm/kvm_page_track.h
> > > +++ b/arch/x86/include/asm/kvm_page_track.h
> > The "#ifdef CONFIG_KVM_EXTERNAL_WRITE_TRACKING" can be moved to the
> > front of this file?
> > All the structures are only exposed for external users now.
> 
> Huh.  I've no idea why I didn't do that.  IIRC, the entire reason past me wrapped
> track_notifier_head in an #ifdef was to allow this change in kvm_page_track.h.
> 
> I'll do this in the next version unless I discover an edge case I'm overlooking.

Ah, deja vu.  I tried this first time around, and got yelled at by the kernel test
robot.  Unsuprisingly, my second attempt yielded the same result :-)

  HDRTEST drivers/gpu/drm/i915/gvt/gvt.h
In file included from <command-line>:
gpu/drivers/gpu/drm/i915/gvt/gvt.h:236:45: error: field ‘track_node’ has incomplete type
  236 |         struct kvm_page_track_notifier_node track_node;
      |                                             ^~~~~~~~~~

The problem is direct header inclusion.  Nothing in the kernel includes gvt.h
when CONFIG_DRM_I915_GVT=n, but the header include guard tests include headers
directly on the command line.  I think I'll define a "stub" specifically to play
nice with this sort of testing.  Guarding the guts of gvt.h with CONFIG_DRM_I915_GVT
would just propagate the problem, and guarding the node definition in "struct
intel_vgpu" would be confusing since the guard would be dead code for all intents
and purposes.

The obvious alternative would be to leave kvm_page_track_notifier_node outside of
the #ifdef, but I really want to bury kvm_page_track_notifier_head for KVM's sake,
and having "head" buried but not "node" would also be weird and confusing.

diff --git a/arch/x86/include/asm/kvm_page_track.h b/arch/x86/include/asm/kvm_page_track.h
index 33f087437209..3d040741044b 100644
--- a/arch/x86/include/asm/kvm_page_track.h
+++ b/arch/x86/include/asm/kvm_page_track.h
@@ -51,6 +51,12 @@ void kvm_page_track_unregister_notifier(struct kvm *kvm,
 
 int kvm_write_track_add_gfn(struct kvm *kvm, gfn_t gfn);
 int kvm_write_track_remove_gfn(struct kvm *kvm, gfn_t gfn);
+#else
+/*
+ * Allow defining a node in a structure even if page tracking is disabled, e.g.
+ * to play nice with testing headers via direct inclusion from the command line.
+ */
+struct kvm_page_track_notifier_node {};
 #endif /* CONFIG_KVM_EXTERNAL_WRITE_TRACKING */
 
 #endif