Re: [RFC PATCH v3 04/19] KVM: x86: mmu: allow to enable write tracking externally

[Maxim Levitsky <mlevitsk@redhat.com>]

On Thu, 2022-05-19 at 16:27 +0000, Sean Christopherson wrote:
> On Wed, Apr 27, 2022, Maxim Levitsky wrote:
> > This will be used to enable write tracking from nested AVIC code
> > and can also be used to enable write tracking in GVT-g module
> > when it actually uses it as opposed to always enabling it,
> > when the module is compiled in the kernel.
> 
> Wrap at ~75.
Well, the checkpatch.pl didn't complain, so I didn't notice.

> 
> > No functional change intended.
> > 
> > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> > ---
> >  arch/x86/include/asm/kvm_host.h       |  2 +-
> >  arch/x86/include/asm/kvm_page_track.h |  1 +
> >  arch/x86/kvm/mmu.h                    |  8 +++++---
> >  arch/x86/kvm/mmu/mmu.c                | 17 ++++++++++-------
> >  arch/x86/kvm/mmu/page_track.c         | 10 ++++++++--
> >  5 files changed, 25 insertions(+), 13 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 636df87542555..fc7df778a3d71 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1254,7 +1254,7 @@ struct kvm_arch {
> >  	 * is used as one input when determining whether certain memslot
> >  	 * related allocations are necessary.
> >  	 */
> 
> The above comment needs to be rewritten.
Good catch, thank a lot!!

> 
> > -	bool shadow_root_allocated;
> > +	bool mmu_page_tracking_enabled;
> >  #if IS_ENABLED(CONFIG_HYPERV)
> >  	hpa_t	hv_root_tdp;
> > diff --git a/arch/x86/include/asm/kvm_page_track.h b/arch/x86/include/asm/kvm_page_track.h
> > index eb186bc57f6a9..955a5ae07b10e 100644
> > --- a/arch/x86/include/asm/kvm_page_track.h
> > +++ b/arch/x86/include/asm/kvm_page_track.h
> > @@ -50,6 +50,7 @@ int kvm_page_track_init(struct kvm *kvm);
> >  void kvm_page_track_cleanup(struct kvm *kvm);
> >  
> >  bool kvm_page_track_write_tracking_enabled(struct kvm *kvm);
> > +int kvm_page_track_write_tracking_enable(struct kvm *kvm);
> >  int kvm_page_track_write_tracking_alloc(struct kvm_memory_slot *slot);
> >  
> >  void kvm_page_track_free_memslot(struct kvm_memory_slot *slot);
> > diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> > index 671cfeccf04e9..44d15551f7156 100644
> > --- a/arch/x86/kvm/mmu.h
> > +++ b/arch/x86/kvm/mmu.h
> > @@ -269,7 +269,7 @@ int kvm_arch_write_log_dirty(struct kvm_vcpu *vcpu);
> >  int kvm_mmu_post_init_vm(struct kvm *kvm);
> >  void kvm_mmu_pre_destroy_vm(struct kvm *kvm);
> >  
> > -static inline bool kvm_shadow_root_allocated(struct kvm *kvm)
> > +static inline bool mmu_page_tracking_enabled(struct kvm *kvm)
> >  {
> >  	/*
> >  	 * Read shadow_root_allocated before related pointers. Hence, threads
> > @@ -277,9 +277,11 @@ static inline bool kvm_shadow_root_allocated(struct kvm *kvm)
> >  	 * see the pointers. Pairs with smp_store_release in
> >  	 * mmu_first_shadow_root_alloc.
> >  	 */
> 
> This comment also needs to be rewritten.
Also thanks a lot, next time I'll check comments better.

> 
> > -	return smp_load_acquire(&kvm->arch.shadow_root_allocated);
> > +	return smp_load_acquire(&kvm->arch.mmu_page_tracking_enabled);
> >  }
> 
> ...
> 
> > diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c
> > index 2e09d1b6249f3..8857d629036d7 100644
> > --- a/arch/x86/kvm/mmu/page_track.c
> > +++ b/arch/x86/kvm/mmu/page_track.c
> > @@ -21,10 +21,16 @@
> >  
> >  bool kvm_page_track_write_tracking_enabled(struct kvm *kvm)
> 
> This can be static, it's now used only by page_track.c.
I'll fix this.
> 
> >  {
> > -	return IS_ENABLED(CONFIG_KVM_EXTERNAL_WRITE_TRACKING) ||
> > -	       !tdp_enabled || kvm_shadow_root_allocated(kvm);
> > +	return mmu_page_tracking_enabled(kvm);
> >  }
> >  
> > +int kvm_page_track_write_tracking_enable(struct kvm *kvm)
> 
> This is too similar to the "enabled" version; "kvm_page_track_enable_write_tracking()"
> would maintain namespacing and be less confusing.
Makes sense, thanks, will do!

> 
> Hmm, I'd probably vote to make this a "static inline" in kvm_page_track.h, and
> rename mmu_enable_write_tracking() to kvm_mmu_enable_write_tracking and export.
> Not a strong preference, just feels silly to export a one-liner.

The sole reason I did it this way, because 'page_track.c' this way contains all the interfaces
that an external user of write tracking needs to use.

> 
> > +{
> > +	return mmu_enable_write_tracking(kvm);
> > +}
> > +EXPORT_SYMBOL_GPL(kvm_page_track_write_tracking_enable);
> > +
> > +
> >  void kvm_page_track_free_memslot(struct kvm_memory_slot *slot)
> >  {
> >  	int i;
> > -- 
> > 2.26.3
> > 

Best regards,
	Maxim Levitsky