From mboxrd@z Thu Jan  1 00:00:00 1970
From: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Subject: Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc
Date: Tue, 26 Nov 2013 11:21:37 +0800
Message-ID: <529413C1.60302@linux.vnet.ibm.com>
References: <1382534973-13197-1-git-send-email-xiaoguangrong@linux.vnet.ibm.com> <1382534973-13197-8-git-send-email-xiaoguangrong@linux.vnet.ibm.com> <20131122191429.GA13308@amt.cnet> <65EE805B-B5DB-4BD0-A057-E5FF78D96D67@linux.vnet.ibm.com> <5292EE2F.5090305@linux.vnet.ibm.com> <20131125181254.GB21858@amt.cnet>
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: QUOTED-PRINTABLE
Cc: Gleb Natapov <gleb@redhat.com>, avi.kivity@gmail.com,
	"pbonzini@redhat.com Bonzini" <pbonzini@redhat.com>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	Eric Dumazet <dada1@cosmosbay.com>,
	Peter Zijlstra <peterz@infradead.org>
To: Marcelo Tosatti <mtosatti@redhat.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20131125181254.GB21858@amt.cnet>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

On 11/26/2013 02:12 AM, Marcelo Tosatti wrote:
> On Mon, Nov 25, 2013 at 02:29:03PM +0800, Xiao Guangrong wrote:
>>>> Also, there is no guarantee of termination (as long as sptes are
>>>> deleted with the correct timing). BTW, can't see any guarantee of
>>>> termination for rculist nulls either (a writer can race with a loc=
kless
>>>> reader indefinately, restarting the lockless walk every time).
>>>
>>> Hmm, that can be avoided by checking dirty-bitmap before rewalk,
>>> that means, if the dirty-bitmap has been set during lockless write-=
protection,
>>> it=EF=BF=BDs unnecessary to write-protect its sptes. Your idea?
>> This idea is based on the fact that the number of rmap is limited by
>> RMAP_RECYCLE_THRESHOLD. So, in the case of adding new spte into rmap=
,
>> we can break the rewalk at once, in the case of deleting, we can onl=
y
>> rewalk RMAP_RECYCLE_THRESHOLD times.
>=20
> Please explain in more detail.

Okay.

My proposal is like this:

pte_list_walk_lockless()
{
restart:

+	if (__test_bit(slot->arch.dirty_bitmap, gfn-index))
+		return;

	code-doing-lockless-walking;
	......
}

Before do lockless-walking, we check the dirty-bitmap first, if
it is set we can simply skip write-protection for the gfn, that
is the case that new spte is being added into rmap when we lockless
access the rmap.

=46or the case of deleting spte from rmap, the number of entry is limit=
ed
by RMAP_RECYCLE_THRESHOLD, that is not endlessly.