From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Daney <ddaney.cavm@gmail.com>
Subject: Q: What is the struct kvm srcu protecting?
Date: Thu, 02 May 2013 11:22:52 -0700
Message-ID: <5182AEFC.9070703@gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
To: kvm@vger.kernel.org
Return-path: <kvm-owner@vger.kernel.org>
Received: from mail-pd0-f176.google.com ([209.85.192.176]:47438 "EHLO
	mail-pd0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1760261Ab3EBSWz (ORCPT <rfc822;kvm@vger.kernel.org>);
	Thu, 2 May 2013 14:22:55 -0400
Received: by mail-pd0-f176.google.com with SMTP id x10so492550pdj.7
        for <kvm@vger.kernel.org>; Thu, 02 May 2013 11:22:55 -0700 (PDT)
Received: from dl.caveonetworks.com (64.2.3.195.ptr.us.xo.net. [64.2.3.195])
        by mx.google.com with ESMTPSA id ts3sm8249001pbc.12.2013.05.02.11.22.53
        for <kvm@vger.kernel.org>
        (version=TLSv1 cipher=RC4-SHA bits=128/128);
        Thu, 02 May 2013 11:22:54 -0700 (PDT)
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

Hi,

I am working on the MIPS KVM port, and am trying to figure out under 
which circumstances do I need to srcu_read_lock()/srcu_read_unlock() the 
kvm->srcu.

I am looking at implementing something similar to arch/x86/kvm/x86.c at 
__msr_io(), where we see:

.
.
.
	idx = srcu_read_lock(&vcpu->kvm->srcu);
	for (i = 0; i < msrs->nmsrs; ++i)
		if (do_msr(vcpu, entries[i].index, &entries[i].data))
			break;
	srcu_read_unlock(&vcpu->kvm->srcu, idx);
.
.
.

Why is the srcu_read_lock() taken here?  I see no srcu_dereference() in 
the code path that would indicate the need for obtaining the lock.

I have a feeling that I am missing some essential concept about the 
design of this code, but I don't know what it is.

Can someone explain what is happening here?


Thanks in advance,
David Daney