From: "Michael S. Tsirkin" <mst@redhat.com>
To: Panu Matilainen <pmatilai@redhat.com>
Cc: dev@dpdk.org, Victor Kaplansky <vkaplans@redhat.com>
Subject: Re: [dpdk-dev] [PATCH 1/4] vhost: handle VHOST_USER_SET_LOG_BASE request
Date: Wed, 2 Dec 2015 19:24:53 +0200 [thread overview]
Message-ID: <20151202192302-mutt-send-email-mst@redhat.com> (raw)
In-Reply-To: <565F231B.5090203@redhat.com>
On Wed, Dec 02, 2015 at 06:58:03PM +0200, Panu Matilainen wrote:
> On 12/02/2015 05:09 PM, Yuanhan Liu wrote:
> >On Wed, Dec 02, 2015 at 04:48:14PM +0200, Panu Matilainen wrote:
> >...
> >>>>>diff --git a/lib/librte_vhost/rte_virtio_net.h b/lib/librte_vhost/rte_virtio_net.h
> >>>>>index 5687452..416dac2 100644
> >>>>>--- a/lib/librte_vhost/rte_virtio_net.h
> >>>>>+++ b/lib/librte_vhost/rte_virtio_net.h
> >>>>>@@ -127,6 +127,8 @@ struct virtio_net {
> >>>>> #define IF_NAME_SZ (PATH_MAX > IFNAMSIZ ? PATH_MAX : IFNAMSIZ)
> >>>>> char ifname[IF_NAME_SZ]; /**< Name of the tap device or socket path. */
> >>>>> uint32_t virt_qp_nb; /**< number of queue pair we have allocated */
> >>>>>+ uint64_t log_size; /**< Size of log area */
> >>>>>+ uint8_t *log_base; /**< Where dirty pages are logged */
> >>>>> void *priv; /**< private context */
> >>>>> struct vhost_virtqueue *virtqueue[VHOST_MAX_QUEUE_PAIRS * 2]; /**< Contains all virtqueue information. */
> >>>>> } __rte_cache_aligned;
> >>>>
> >>>>This (and other changes in patch 2 breaks the librte_vhost ABI
> >>>>again, so you'd need to at least add a deprecation note to 2.2 to be
> >>>>able to do it in 2.3 at all according to the ABI policy.
> >>>
> >>>I was thinking that adding a new field (instead of renaming it or
> >>>removing it) isn't an ABI break. So, I was wrong?
> >>
> >>Adding or removing a field in the middle of a public struct is
> >>always an ABI break. Adding to the end often is too, but not always.
> >>Renaming a field is an API break but not an ABI break - the compiler
> >>cares but the cpu does not.
> >
> >Good to know. Thanks.
> >
> >>
> >>>>
> >>>>Perhaps a better option would be adding some padding to the structs
> >>>>now for 2.2 since the vhost ABI is broken there anyway. That would
> >>>>at least give a chance to keep it compatible from 2.2 to 2.3.
> >>>
> >>>It will not be compatible, unless we add exact same fields (not
> >>>something like uint8_t pad[xx]). Otherwise, the pad field renaming
> >>>is also an ABI break, right?
> >>
> >>There's no ABI (or API) break in changing reserved unused fields to
> >>something else, as long as care is taken with sizes and alignment.
> >
> >as long as we don't reference the reserved unused fields?
>
> That would be the definition of an unused field I think :)
> Call it "reserved" if you want, it doesn't really matter as long as its
> clear its something you shouldn't be using.
>
> >
> >>In any case padding is best added to the end of a struct to minimize
> >>risks and keep things simple.
> >
> >The thing is that isn't it a bit aweful to (always) add pads to
> >the end of a struct, especially when you don't know how many
> >need to be padded?
>
> Then you pad for what you think you need, plus a bit extra, and maybe some
> more for others who might want to extend it. What is a reasonable amount
> needs deciding case by case - if a struct is alloced in the millions then be
> (very) conservative, but if there are one or 50 such structs within an app
> lifetime then who cares if its bit larger?
>
> And yeah padding may be annoying, but that's pretty much the only option in
> a project where most of the structs are out in the open.
>
> - Panu -
Functions versioning is another option.
For a sufficiently widely used struct, it's a lot of work, padding
is easier. But it might be better than breaking ABI
if e.g. you didn't pad enough.
> >
> > --yliu
> >
next prev parent reply other threads:[~2015-12-02 17:24 UTC|newest]
Thread overview: 98+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-12-02 3:43 [dpdk-dev] [PATCH 0/4 for 2.3] vhost-user live migration support Yuanhan Liu
2015-12-02 3:43 ` [dpdk-dev] [PATCH 1/4] vhost: handle VHOST_USER_SET_LOG_BASE request Yuanhan Liu
2015-12-02 13:53 ` Panu Matilainen
2015-12-02 14:31 ` Yuanhan Liu
2015-12-02 14:48 ` Panu Matilainen
2015-12-02 15:09 ` Yuanhan Liu
2015-12-02 16:58 ` Panu Matilainen
2015-12-02 17:24 ` Michael S. Tsirkin [this message]
2015-12-02 16:38 ` Thomas Monjalon
2015-12-03 1:49 ` Yuanhan Liu
2015-12-06 23:07 ` Thomas Monjalon
2015-12-07 2:00 ` Yuanhan Liu
2015-12-07 2:03 ` Thomas Monjalon
2015-12-07 2:18 ` Yuanhan Liu
2015-12-07 2:49 ` Thomas Monjalon
2015-12-07 6:29 ` Panu Matilainen
2015-12-07 11:28 ` Thomas Monjalon
2015-12-07 11:41 ` Panu Matilainen
2015-12-07 13:55 ` Thomas Monjalon
2015-12-07 16:48 ` Panu Matilainen
2015-12-07 17:47 ` Thomas Monjalon
2015-12-08 5:57 ` Xie, Huawei
2015-12-08 7:25 ` Yuanhan Liu
2015-12-02 3:43 ` [dpdk-dev] [PATCH 2/4] vhost: introduce vhost_log_write Yuanhan Liu
2015-12-02 13:53 ` Victor Kaplansky
2015-12-02 14:39 ` Yuanhan Liu
2015-12-09 3:33 ` Xie, Huawei
2015-12-09 3:42 ` Yuanhan Liu
2015-12-09 5:44 ` Xie, Huawei
2015-12-09 8:41 ` Yuanhan Liu
2015-12-02 3:43 ` [dpdk-dev] [PATCH 3/4] vhost: log vring changes Yuanhan Liu
2015-12-02 14:07 ` Victor Kaplansky
2015-12-02 14:38 ` Yuanhan Liu
2015-12-02 15:58 ` Victor Kaplansky
2015-12-02 16:26 ` Michael S. Tsirkin
2015-12-03 2:31 ` Yuanhan Liu
2015-12-09 2:45 ` Xie, Huawei
2015-12-02 3:43 ` [dpdk-dev] [PATCH 4/4] vhost: enable log_shmfd protocol feature Yuanhan Liu
2015-12-02 14:10 ` [dpdk-dev] [PATCH 0/4 for 2.3] vhost-user live migration support Victor Kaplansky
2015-12-02 14:33 ` Yuanhan Liu
2015-12-09 3:41 ` Xie, Huawei
2015-12-17 3:11 ` [dpdk-dev] [PATCH v2 0/6] " Yuanhan Liu
2015-12-17 3:11 ` [dpdk-dev] [PATCH v2 1/6] vhost: handle VHOST_USER_SET_LOG_BASE request Yuanhan Liu
2015-12-21 15:32 ` Xie, Huawei
2015-12-22 2:25 ` Yuanhan Liu
2015-12-22 2:41 ` Xie, Huawei
2015-12-22 2:55 ` Yuanhan Liu
2015-12-17 3:11 ` [dpdk-dev] [PATCH v2 2/6] vhost: introduce vhost_log_write Yuanhan Liu
2015-12-21 15:06 ` Xie, Huawei
2015-12-22 2:40 ` Yuanhan Liu
2015-12-22 2:45 ` Xie, Huawei
2015-12-22 3:04 ` Yuanhan Liu
2015-12-22 7:02 ` Xie, Huawei
2015-12-22 5:11 ` Peter Xu
2015-12-22 6:09 ` Yuanhan Liu
2015-12-17 3:11 ` [dpdk-dev] [PATCH v2 3/6] vhost: log used vring changes Yuanhan Liu
2015-12-22 6:55 ` Peter Xu
2015-12-22 7:07 ` Xie, Huawei
2015-12-22 7:59 ` Peter Xu
2015-12-22 7:13 ` Yuanhan Liu
2015-12-22 8:01 ` Peter Xu
2015-12-17 3:11 ` [dpdk-dev] [PATCH v2 4/6] vhost: log vring desc buffer changes Yuanhan Liu
2015-12-17 3:12 ` [dpdk-dev] [PATCH v2 5/6] vhost: claim that we support GUEST_ANNOUNCE feature Yuanhan Liu
2015-12-22 8:11 ` Peter Xu
2015-12-22 8:21 ` Yuanhan Liu
2015-12-22 8:24 ` Pavel Fedin
2015-12-17 3:12 ` [dpdk-dev] [PATCH v2 6/6] vhost: enable log_shmfd protocol feature Yuanhan Liu
2015-12-17 12:08 ` [dpdk-dev] [PATCH v2 0/6] vhost-user live migration support Iremonger, Bernard
2015-12-17 12:45 ` Yuanhan Liu
2015-12-21 8:17 ` Pavel Fedin
2016-01-29 4:57 ` [dpdk-dev] [PATCH v3 0/8] " Yuanhan Liu
2016-01-29 4:57 ` [dpdk-dev] [PATCH v3 1/8] vhost: handle VHOST_USER_SET_LOG_BASE request Yuanhan Liu
2016-01-29 4:57 ` [dpdk-dev] [PATCH v3 2/8] vhost: introduce vhost_log_write Yuanhan Liu
2016-02-19 14:26 ` Thomas Monjalon
2016-02-22 6:59 ` Yuanhan Liu
2016-01-29 4:57 ` [dpdk-dev] [PATCH v3 3/8] vhost: log used vring changes Yuanhan Liu
2016-01-29 4:57 ` [dpdk-dev] [PATCH v3 4/8] vhost: log vring desc buffer changes Yuanhan Liu
2016-01-29 4:58 ` [dpdk-dev] [PATCH v3 5/8] vhost: claim that we support GUEST_ANNOUNCE feature Yuanhan Liu
2016-03-11 12:39 ` Olivier MATZ
2016-03-11 13:16 ` Thomas Monjalon
2016-03-11 13:22 ` Olivier MATZ
2016-01-29 4:58 ` [dpdk-dev] [PATCH v3 6/8] vhost: handle VHOST_USER_SEND_RARP request Yuanhan Liu
2016-02-19 6:11 ` Tan, Jianfeng
2016-02-19 7:03 ` Yuanhan Liu
2016-02-19 8:55 ` Yuanhan Liu
2016-02-22 14:36 ` [dpdk-dev] [PATCH] vhost: broadcast RARP pkt by injecting it to receiving mbuf array Yuanhan Liu
2016-02-24 8:15 ` Qiu, Michael
2016-02-24 8:28 ` Yuanhan Liu
2016-02-25 7:55 ` Qiu, Michael
2016-02-29 15:56 ` Thomas Monjalon
2016-01-29 4:58 ` [dpdk-dev] [PATCH v3 7/8] vhost: enable log_shmfd protocol feature Yuanhan Liu
2016-01-29 4:58 ` [dpdk-dev] [PATCH v3 8/8] vhost: remove duplicate header include Yuanhan Liu
2016-02-01 15:54 ` [dpdk-dev] [PATCH v3 0/8] vhost-user live migration support Iremonger, Bernard
2016-02-02 1:53 ` Yuanhan Liu
2016-02-19 15:01 ` Thomas Monjalon
2016-02-22 7:08 ` Yuanhan Liu
2016-02-22 9:56 ` Thomas Monjalon
2016-02-22 14:24 ` Yuanhan Liu
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