Re: [Qemu-devel] [PATCH 4/4] KVM: MMU: fast cleanup D bit based on fast write protect

[Sean Christopherson]

On Mon, Jan 21, 2019 at 06:37:36AM +0000, Zhuangyanying wrote:
> 
> > >   u64 wp_all_indicator, kvm_wp_all_gen;
> > >
> > > - mutex_lock(&kvm->slots_lock);
> > >   wp_all_indicator = get_write_protect_all_indicator(kvm);
> > >   kvm_wp_all_gen = get_write_protect_all_gen(wp_all_indicator);
> > >
> > > @@ -6134,8 +6136,8 @@ void kvm_mmu_write_protect_all_pages(struct
> > kvm *kvm, bool write_protect)
> > >    */
> > >   if (write_protect)
> > >           kvm_reload_remote_mmus(kvm);
> > > - mutex_unlock(&kvm->slots_lock);
> > 
> > Why is the lock removed?  And why was it added in the first place?
> > 
> The original purpose of fast write protect is to implement lock-free 
> get_dirty_log,
> kvm_mmu_write_protect_all_pages is a stand-alone kvm API. See
> https://lists.gnu.org/archive/html/qemu-devel/2017-06/msg04370.html
> A total of 7 patches, I only need the first 3 patches to achieve step-by-step
> page table traversal. In order to maintain the integrity of the xiaoguangrong
> patch, I did not directly modify it on his patch.

That's not a sufficient argument for adding locking and removing it one
patch later.

> > >  }
> > > +EXPORT_SYMBOL_GPL(kvm_mmu_write_protect_all_pages);
> > >
> > >  static unsigned long
> > >  mmu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index
> > > f6915f1..5236a07 100644
> > > --- a/arch/x86/kvm/vmx/vmx.c
> > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > @@ -7180,8 +7180,7 @@ static void vmx_sched_in(struct kvm_vcpu *vcpu,
> > > int cpu)  static void vmx_slot_enable_log_dirty(struct kvm *kvm,
> > >                                struct kvm_memory_slot *slot)  {
> > > - kvm_mmu_slot_leaf_clear_dirty(kvm, slot);
> > > - kvm_mmu_slot_largepage_remove_write_access(kvm, slot);
> > > + kvm_mmu_write_protect_all_pages(kvm, true);
> > 
> > What's the purpose of having @write_protect if
> > kvm_mmu_write_protect_all_pages() is only ever called to enable
> > protection?  If there's no known scenario where write protection is
> > explicitly disabled then there's no need for WP_ALL_ENABLE_MASK, i.e. a
> > non-zero generation would indicate write protection is enabled.  That'd
> > simplify the code and clean up the atomic usage.
> > 
> In the live migration, The large page split depends on the creation of 
> memslot->dirty_bitmap
> in the function __kvm_set_memory_region().
> The interface design between qemu and kvm to enable dirty log is one by one 
> in slot units.
> In order to enable dirty page tracking of the entire vm, it is necessary to 
> call
> kvm_mmu_write_protect_all_pages multiple times. The page table update request 
> can
> be merged for processing by the atomic usage. This method is not elegant, but 
> it works.
> Complete the creation of all solt's dirty_bitmap in an API, just call 
> kvm_mmu_write_protect_all_pages once, need more implementation changes, even 
> qemu.

Calling kvm_mmu_write_protect_all_pages() multiple times is fine.  My
question was regarding the 'write_protect' parameter.  If all callers
always pass %true for 'write_protect' then why does the parameter exist?
And eliminating the parameter means you don't need an 'enable' flag
buried in the generation, which would simplify the implementation.