linux-kernel - Re: [PATCH] KVM: Remove redundant smp_mb() in the kvm_mmu_commit_zap

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Date:	Fri, 11 Mar 2016 02:08:05 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Lan Tianyu <tianyu.lan@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	gleb@...nel.org, mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: Remove redundant smp_mb() in the
 kvm_mmu_commit_zap_page()



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.