Re: [PATCH] KVM: Remove redundant smp_mb() in the kvm_mmu_commit_zap_page()

[Xiao Guangrong]

On 03/11/2016 01:07 AM, Paolo Bonzini wrote:
> On 09/03/2016 08:18, Lan Tianyu wrote:
> > How about the following comments.
> >
> > Log for kvm_mmu_commit_zap_page()
> > 	/*
> > 	 * We need to make sure everyone sees our modifications to
> > 	 * the page tables and see changes to vcpu->mode here.
>
> Please mention that this pairs with vcpu_enter_guest and
> walk_shadow_page_lockless_begin/end.
>
> > The
> > 	 * barrier in the kvm_flush_remote_tlbs() helps us to achieve
> > 	 * these. Otherwise, wait for all vcpus to exit guest mode
> > 	 * and/or lockless shadow page table walks.
> > 	 */
> > 	kvm_flush_remote_tlbs(kvm);
>
> The rest of the comment is okay, but please replace "Otherwise" with "In
> addition, we need to".
>
> > Log for kvm_flush_remote_tlbs()
> > 	/*
> > 	 * We want to publish modifications to the page tables before
> > 	 * reading mode. Pairs with a memory barrier in arch-specific
> > 	 * code.
> > 	 * - x86: smp_mb__after_srcu_read_unlock in vcpu_enter_guest.
>
> ... and smp_mb in walk_shadow_page_lockless_begin/end.
>
> > 	 * - powerpc: smp_mb in kvmppc_prepare_to_enter.
> > 	 */
> >   	 smp_mb__before_atomic();
>
> The comment looks good, but the smp_mb__before_atomic() is not needed.
> As mentioned in the reply to Guangrong, only a smp_load_acquire is required.
>
> So the comment should say something like "There is already an smp_mb()
> before kvm_make_all_cpus_request reads vcpu->mode.  We reuse that
> barrier here.".
>
> On top of this there is:
>
> - the change to paging_tmpl.h that Guangrong posted, adding smp_wmb()
> before each increment of vcpu->kvm->tlbs_dirty

Yes, please make it as a separated patch.

> - the change to smp_mb__after_atomic() in kvm_make_all_cpus_request
>
> - if you want :) you can also replace the store+mb in
> walk_shadow_page_lockless_begin with smp_store_mb, and the mb+store in
> walk_shadow_page_lockless_end with smp_store_release.

These changes are good to me.

TianYu, please CC me when you post the new version out.