From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:47577) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UhbEB-0002oI-Mk for qemu-devel@nongnu.org; Wed, 29 May 2013 03:55:28 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UhbE6-0003Pr-Eb for qemu-devel@nongnu.org; Wed, 29 May 2013 03:55:23 -0400 Received: from e23smtp09.au.ibm.com ([202.81.31.142]:59721) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UhbE5-0003PB-SR for qemu-devel@nongnu.org; Wed, 29 May 2013 03:55:18 -0400 Received: from /spool/local by e23smtp09.au.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 30 May 2013 04:52:09 +1000 Received: from d23relay04.au.ibm.com (d23relay04.au.ibm.com [9.190.234.120]) by d23dlp03.au.ibm.com (Postfix) with ESMTP id DF78C357804E for ; Wed, 29 May 2013 17:54:57 +1000 (EST) Received: from d23av02.au.ibm.com (d23av02.au.ibm.com [9.190.235.138]) by d23relay04.au.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id r4T7eeP027590900 for ; Wed, 29 May 2013 17:40:41 +1000 Received: from d23av02.au.ibm.com (loopback [127.0.0.1]) by d23av02.au.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id r4T7su7F028178 for ; Wed, 29 May 2013 17:54:56 +1000 Message-ID: <51A5B446.7080905@linux.vnet.ibm.com> Date: Wed, 29 May 2013 15:54:46 +0800 From: Wenchao Xia MIME-Version: 1.0 References: <1369451385-23452-1-git-send-email-xiawenc@linux.vnet.ibm.com> <1369451385-23452-2-git-send-email-xiawenc@linux.vnet.ibm.com> <20130527152516.GF2373@dhcp-200-207.str.redhat.com> <20130528074649.GC13368@stefanha-thinkpad.redhat.com> In-Reply-To: <20130528074649.GC13368@stefanha-thinkpad.redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Subject: Re: [Qemu-devel] [PATCH V3 1/4] block: drop bs_snapshots global variable List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Kevin Wolf , phrdina@redhat.com, stefanha@gmail.com, qemu-devel@nongnu.org, lcapitulino@redhat.com, pbonzini@redhat.com, armbru@redhat.com 于 2013-5-28 15:46, Stefan Hajnoczi 写道: > On Mon, May 27, 2013 at 05:25:16PM +0200, Kevin Wolf wrote: >> Am 25.05.2013 um 05:09 hat Wenchao Xia geschrieben: >>> From: Stefan Hajnoczi >>> >>> The bs_snapshots global variable points to the BlockDriverState which >>> will be used to save vmstate. This is really a savevm.c concept but was >>> moved into block.c:bdrv_snapshots() when it became clear that hotplug >>> could result in a dangling pointer. >>> >>> While auditing the block layer's global state I came upon bs_snapshots >>> and realized that a variable is not necessary here. Simply find the >>> first BlockDriverState capable of internal snapshots each time this is >>> needed. >>> >>> The behavior of bdrv_snapshots() is preserved across hotplug because new >>> drives are always appended to the bdrv_states list. This means that >>> calling the new find_vmstate_bs() function is idempotent - it returns >>> the same BlockDriverState unless it was hot-unplugged. >>> >>> Signed-off-by: Stefan Hajnoczi >>> Reviewed-by: Eric Blake >>> Reviewed-by: Wenchao Xia >>> Signed-off-by: Wenchao Xia >>> --- >>> block.c | 28 ---------------------------- >>> include/block/block.h | 1 - >>> savevm.c | 19 +++++++++++++++---- >>> 3 files changed, 15 insertions(+), 33 deletions(-) >>> >>> diff --git a/block.c b/block.c >>> index 3f87489..478a3b2 100644 >>> --- a/block.c >>> +++ b/block.c >>> @@ -99,9 +99,6 @@ static QTAILQ_HEAD(, BlockDriverState) bdrv_states = >>> static QLIST_HEAD(, BlockDriver) bdrv_drivers = >>> QLIST_HEAD_INITIALIZER(bdrv_drivers); >>> >>> -/* The device to use for VM snapshots */ >>> -static BlockDriverState *bs_snapshots; >>> - >>> /* If non-zero, use only whitelisted block drivers */ >>> static int use_bdrv_whitelist; >>> >>> @@ -1357,9 +1354,6 @@ void bdrv_close(BlockDriverState *bs) >>> notifier_list_notify(&bs->close_notifiers, bs); >>> >>> if (bs->drv) { >>> - if (bs == bs_snapshots) { >>> - bs_snapshots = NULL; >>> - } >>> if (bs->backing_hd) { >>> bdrv_delete(bs->backing_hd); >>> bs->backing_hd = NULL; >>> @@ -1591,7 +1585,6 @@ void bdrv_delete(BlockDriverState *bs) >>> >>> bdrv_close(bs); >>> >>> - assert(bs != bs_snapshots); >>> g_free(bs); >>> } >>> >>> @@ -1635,9 +1628,6 @@ void bdrv_set_dev_ops(BlockDriverState *bs, const BlockDevOps *ops, >>> { >>> bs->dev_ops = ops; >>> bs->dev_opaque = opaque; >>> - if (bdrv_dev_has_removable_media(bs) && bs == bs_snapshots) { >>> - bs_snapshots = NULL; >>> - } >> >> This hunk isn't replaced by any other code. If I understand correctly >> what it's doing, it prevented you from saving the VM state to a >> removable device, which would be allowed after this patch. >> >> Is this a change we want to make? Why isn't it described in the commit >> message? > > My understanding of this change is different. Markus is on CC so maybe > he can confirm. > > The point of bs_snapshots = NULL is not to prevent you from saving > snapshots. It's simply to reset the pointer to the next snapshottable > device (used by bdrv_snapshots()). > > See the bdrv_close() hunk above which does the same thing, as well as > bdrv_snapshots() which iterates bdrv_states and updates bs_snapshots. > > So what this hunk does is to reset the bdrv_snapshots() iterator when a > removable device is hooked up to an emulated storage controller. It's > no longer necessary since this patch drops the global state > (bs_snapshots) and users will always iterate from scratch. > > The whole stateful approach was not necessary. > > Stefan > Reading the code, original logic actually forbidded saving vmstate into a removable device, now it is possible since find_vmstate_bs() doesn't check it. How about forbid again in find_vmstate_bs()? -- Best Regards Wenchao Xia