linux-kernel - Re: [PATCH v2 19/22] KVM: MMU: lockless walking shadow page table

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Date:	Wed, 29 Jun 2011 12:16:54 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 19/22] KVM: MMU: lockless walking shadow page table

On 06/22/2011 05:35 PM, Xiao Guangrong wrote:
> Use rcu to protect shadow pages table to be freed, so we can safely walk it,
> it should run fastly and is needed by mmio page fault
>

>   static void kvm_mmu_commit_zap_page(struct kvm *kvm,
>   				    struct list_head *invalid_list)
>   {
> @@ -1767,6 +1874,14 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
>
>   	kvm_flush_remote_tlbs(kvm);
>
> +	if (atomic_read(&kvm->arch.reader_counter)) {
> +		kvm_mmu_isolate_pages(invalid_list);
> +		sp = list_first_entry(invalid_list, struct kvm_mmu_page, link);
> +		list_del_init(invalid_list);
> +		call_rcu(&sp->rcu, free_pages_rcu);
> +		return;
> +	}
> +

I think we should do this unconditionally.  The cost of ping-ponging the 
shared cache line containing reader_counter will increase with large smp 
counts.  On the other hand, zap_page is very rare, so it can be a little 
slower.  Also, less code paths = easier to understand.

-- 
error compiling committee.c: too many arguments to function

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