Re: [PATCH V2 2/3] vdpa_sim_net: support feature provisioning

From: Si-Wei Liu
Date: Thu Oct 06 2022 - 20:35:44 EST



On 9/28/2022 9:55 PM, Jason Wang wrote:
On Tue, Sep 27, 2022 at 5:41 PM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> wrote:


On 9/26/2022 8:59 PM, Jason Wang wrote:

On Tue, Sep 27, 2022 at 9:02 AM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> wrote:


On 9/26/2022 12:11 AM, Jason Wang wrote:

On Sat, Sep 24, 2022 at 4:01 AM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> wrote:


On 9/21/2022 7:43 PM, Jason Wang wrote:

This patch implements features provisioning for vdpa_sim_net.

1) validating the provisioned features to be a subset of the parent
features.
2) clearing the features that is not wanted by the userspace

For example:

# vdpa mgmtdev show
vdpasim_net:
supported_classes net
max_supported_vqs 3
dev_features MTU MAC CTRL_VQ CTRL_MAC_ADDR ANY_LAYOUT VERSION_1 ACCESS_PLATFORM

Sighs, not to blame any one and it's perhaps too late, but this
"dev_features" attr in "mgmtdev show" command output should have been
called "supported_features" in the first place.

Not sure I get this, but I guess this is the negotiated features actually.

Actually no, that is why I said the name is a bit confusing and "supported_features" might sound better.

You're right, it's an mgmtdev show actually.

This attribute in the parent device (mgmtdev) denotes the real device capability for what virtio features can be supported by the parent device. Any unprivileged user can check into this field to know parent device's capability without having to create a child vDPA device at all. The features that child vDPA device may support should be a subset of, or at most up to what the parent device offers. For e.g. the vdpa device dev1 you created below can expose less or equal device_features bit than 0x308820028 (MTU MAC CTRL_VQ CTRL_MAC_ADDR ANY_LAYOUT VERSION_1 ACCESS_PLATFORM), but shouldn't be no more than what the parent device can actually support.

Yes, I didn't see anything wrong with "dev_features",

Yep, it didn't appear to me anything wrong either at first sight, then I gave my R-b on the series introduced this attribute. But it's not a perfect name, either, on the other hand. Parav later pointed out that the corresponding enum definition for this attribute should follow pre-existing naming convention that we should perhaps do s/VDPA_ATTR_DEV_SUPPORTED_FEATURES/VDPA_ATTR_MGMTDEV_SUPPORTED_FEATURES/ to get it renamed, as this is a mgmtdev level attribute, which I agree. Now that with the upcoming "device_features" attribute (vdpa dev level) from this series, it's subject to another confusions between these two similar names, but actually would represent things at different level. While all other attributes in "mgmtdev dev show" seem to be aligned with the "supported_" prefix, e.g. supported_classes, max_supported_vqs, from which I think the stance of device is already implied through "mgmtdev" in the command. For the perspective of clarify and easy distinction, "supported_features" seems to be a better name than "dev_features".
See another reply, I think I get your point,

1) VDPA_ATTR_VDPA_DEV_SUPPORTED_FEATURES (lingshan's series) and
VDPA_ATTR_VDPA_DEV_FEATURES should be equivalent and unified to a
single attribute.
2) A better name to "supported_features" should be fine, patches are welcomed

it aligns to the
virtio spec which means the features could be used to create a vdpa
device. But if everyone agree on the renaming, I'm fine.

Never mind, if it's late don't have to bother.



I think Ling Shan is working on reporting both negotiated features
with the device features.

Does it imply this series is connected to another work in parallel? Is it possible to add a reference in the cover letter?

I'm not sure, I remember Ling Shan did some work to not block the
config show in this commit:

commit a34bed37fc9d3da319bb75dfbf02a7d3e95e12de
Author: Zhu Lingshan <lingshan.zhu@xxxxxxxxx>
Date: Fri Jul 22 19:53:07 2022 +0800

vDPA: !FEATURES_OK should not block querying device config space

We need some changes in the vdpa tool to show device_features
unconditionally in the "dev config show" command.

That's true, I think I ever pointed it out to Lingshan before, that it's not needed to bother exposing those config space fields in "dev config show" output, if the only intent is for live migration of device features between nodes. For vDPA live migration, what cares most is those configuration parameters specified on vdpa creation, and userspace VMM (QEMU) is supposed to take care of saving and restoring live device states. I think it's easier to extend "vdpa dev show" output to include device_features and other config params as well, rather than count on validity of various config space fields.
Probably, but for the migration it's more about the ability of the
mgmtdev instead of the vDPA device itself I think.
If picking the appropriate device for migration is what it is concerned, it's subject to the capability that mgmtdev offers. That's true, for sure.

On the other hand, mgmt software would also need to take care of reconstructing the destination device with the same configuration as that at the source side. For example, a mgmtdev on source supports features A, B, C, D,  and destination mgmtdev supports features B, C, D, E. When vdpa device on the source is initially created with features B and C but without feature D (noted: creation with a subset of mgmtdev features was already supported before, and this series just makes it more explicit), the mgmt software is supposed to equally create a device with features B and C on dest, even though the destination may support feature D that the mgmtdev on both sides can support. My point is, we should have some flexibility on the mgmt software implementation that allows the mgmt software to easily tell apart the exact features (i.e. B and C in the above example) and the exact configuration a specific vdpa device was originally created with, via some simple query command. Having mgmt software to remember the vdpa creation args could work, but on the other hand it'd be nice to allow lightweight software implementation without having to persist the list of vdpa args and make vdpa tool more self-contained.

I will post a patch (series) shortly to demonstrate this idea. Basically, there's no actual need to mess around with those config space values for live migration. It was not built for that purpose.


https://lore.kernel.org/virtualization/454bdf1b-daa1-aa67-2b8c-bc15351c1851@xxxxxxxxxx/

It's not just insufficient, but sometimes is incorrect to create vDPA device using the config space fields. For instance, MAC address in config space can be changed temporarily (until device reset) via ctrl_vq VIRTIO_NET_CTRL_MAC_ADDR_SET command. It's incorrect to create vDPA using the MAC address shown in the config space.
I think it's still a must for create the mac with the exact mac address:

1) VIRTIO_NET_F_CTRL_MAC is not a must
2) there's no way for us to know whether or not the mac has been changed
Noted I think here we are still talking about VERSION_1 device which is spec conforming. So far as I understand the spec, if the VIRTIO_NET_F_CTRL_MAC feature is not negotiated, there's no way for driver to change the default MAC address.

Even if we want to simulate or support a legacy device model, when MAC address is changed by legacy driver somehow, QEMU should be able to detect this and issue a vdpa ioctl call to change the MAC address filter underneath. I don't see that it ever happens in today's code, so I presume the only possibility this may work is that the specific vDPA device may have an internal learning bridge that adapts to what MAC address the driver actually sends. In this case, since the device doesn't care, recreate with the MAC in use is not needed, and technically it is even incorrect. In data centers or cloud environment, MAC address is usually controlled and managed by some central entity/service. If a  driver can dominate the MAC address in use by deliberately overriding the default MAC and bypassing the central rule via live migration, that'd be a more severe security issue to address in the first place.

3) migration code can restore the mac during restore

So exactly the same mac address is still required. (This is the same
as we are doing for live migration on software virtio)

Another example, if the source vDPA device has MAC address table size limit of 100, then in the destination we should pick parent device with size limit no smaller than that, and create vDPA on remote node matching the exact same size. There's nothing config space field can assist here.
Two ways:

1) mgmtdev should show the mac table size so mgmt layer can provision
the mac table size
2) If the mac table exceeds what is supported in the destination, it
needs to enable the all uni in this case.
Yep, so no field in the config space can help with these two solutions, right? MAC table size is not showing up there. Whether the parent device supports ALLUNI via VIRTIO_NET_F_CTRL_RX is not there, either. (showing them in the 'vdpa mgmtdev show' output is the right thing IMHO).


One example further, in the future, if we are going to introduce mandatory feature (for e.g. VERSION_1, RING_PACKED) that the device is unable to support the opposite case, the destination device should be created with equally same mandatory device features, which only vDPA creation parameters should matter. While I can't think of a case that the mgmt software or live migration tool would have to count on config space fields only.
Yes, in this case we need to introduce new netlink attributes for both
getting mandatory features from the management device and provisioning
those manadating features.
True, management device level thing again, not related to anything in the config space.





1) provision vDPA device with all features that are supported by the
net simulator

# vdpa dev add name dev1 mgmtdev vdpasim_net
# vdpa dev config show
dev1: mac 00:00:00:00:00:00 link up link_announce false mtu 1500
negotiated_features MTU MAC CTRL_VQ CTRL_MAC_ADDR VERSION_1 ACCESS_PLATFORM

Maybe not in this patch, but for completeness for the whole series,
could we also add device_features to the output?

Lingshan, could you please share your thoughts or patch on this?

Noted here the device_features argument specified during vdpa creation is introduced by this series itself, it somehow slightly changed the original semantics of what device_features used to be.

I'm not sure I get this, we don't support device_features in the past
and it is used to provision device features to the vDPA device which
seems to be fine.

Before this change, only look at the dev_features in "mgmtdev show" and remember creation parameters is sufficient to get to all needed info for creating vDPA at destination.
Note that even with the same vendor, mgmtdev may support different features.

After this change, dev_features in "mgmtdev show" becomes less relevant, as it would need to remember vdpa creation parameters plus the device_features attribute. While this series allows cross vendor live migration, it would complicate the implementation of mgmt software, on the other hand.
To allow cross vendor live migration I couldn't find a better way.
The idea itself is great, I think, though the CLI interface may have some space for improvement. For example, user has to supply the heximal value consisting of each feature bit, which is a bit challenging for normal users who are not super familiar with each virtio feature. On the other hand, there could be ambiguity against other vdpa create option, e.g. users may do "vdpa dev add name vdpa0 mgmtdev ... mtu 1500 device_features 0x300020000" (no F_MTU feature bit in device_features) that needs special validation in the code.

How about we follow the CPU flags model or QEMU virtio-net-pci args to define property representing each feature bit? I think the current convention for each "vdpa dev add" option implies that the corresponding feature bit will be enabled once specified in creation. Similarly we can introduce additional option representing each feature bit, along with a new features_default property denoting the initial value for device_feature bits:

# vdpa mgmtdev show
vdpasim_net:
supported_classes net
max_supported_vqs 3
dev_features MTU MAC CTRL_VQ CTRL_MAC_ADDR ANY_LAYOUT VERSION_1 ACCESS_PLATFORM
# vdpa dev add name dev1 mgmtdev vdpasim_net features_default off \
csum on guest_csum on mtu 2000 ctrl_vq on version1 on access_platform on
# vdpa dev show
dev1: type network mgmtdev vdpasim_net vendor_id 0 max_vqs 3 max_vq_size 256
features_default off mtu 2000
device_features CSUM GUEST_CSUM MTU CTRL_VQ VERSION_1 ACCESS_PLATFORM

If the features_default property is left unspecified or with the "inherited" value, it would just inherit all of the supported_features from mgmtdev (which is already the case of today).

# vdpa dev add name dev1 mgmtdev vdpasim_net features_default inherited
# vdpa dev show
dev1: type network mgmtdev vdpasim_net vendor_id 0 max_vqs 3 max_vq_size 256
features_default inherited
device_features MTU MAC CTRL_VQ CTRL_MAC_ADDR ANY_LAYOUT VERSION_1 ACCESS_PLATFORM

I can definitely help implement this model if you find it fits.




When simply look at the "vdpa dev config show" output, I cannot really
tell the actual device_features that was used in vdpa creation. For e.g.
there is a missing feature ANY_LAYOUT from negotiated_features compared
with supported_features in mgmtdev, but the orchestration software
couldn't tell if the vdpa device on destination host should be created
with or without the ANY_LAYOUT feature.

I think VERSION_1 implies ANY_LAYOUT.

Right, ANY_LAYOUT is a bad example. A good example might be that, I knew the parent mgmtdev on migration source node supports CTRL_MAC_ADDR, but I don't find it in negotiated_features.

I think we should use the features that we got from "mgmtdev show"
instead of "negotiated features".

That was how it's supposed to work previously, but with this series, I think the newly introduced device_features will be needed instead of the one in "mgmtdev show".
Just to clarify, there won't be a device_features in mgmtdev show
since it is device specific, each individual device can have its own
device features which are subset of what is supported by the mgmtdev.
Yep.


On the migration destination node, the parent device does support all features as the source offers, including CTRL_MAC_ADDR. What device features you would expect the mgmt software to create destination vdpa device with, if not otherwise requiring mgmt software to remember all the arguments on device creation?
So the provisioning in the destination should use exactly the same
device_feautres as what the vdpa device has in the source. But before
this, management layer should guarantee to provision a vDPA device
whose device_features can be supported on the destination in order to
allow live migration.
Exactly.

So in this example, we need use "dev_features" so we get exact the
same features after and operation as either src or dst.

If the device_features vDPA created with at the source doesn't include CTRL_MAC_ADDR even though parent supports it, then the vDPA to be created at the destination shouldn't come with CTRL_MAC_ADDR either, regardless of whether or not CTRL_MAC_ADDR is present in destination "mgmtdev show".

However, if just taking look at negotiated_features, some mgmt software implementations which don't persist the creation parameters can't get the device features a certain vDPA device at the source node was created with.


SOURCE# vdpa mgmtdev show
vdpasim_net:
supported_classes net
max_supported_vqs 3
dev_features MTU MAC CTRL_VQ CTRL_MAC_ADDR ANY_LAYOUT VERSION_1 ACCESS_PLATFORM
SOURCE# vdpa dev config show
dev1: mac 00:00:00:00:00:00 link up link_announce false mtu 1500
negotiated_features MTU MAC CTRL_VQ VERSION_1 ACCESS_PLATFORM

DESTINATION# vdpa mgmtdev show
vdpasim_net:
supported_classes net
max_supported_vqs 3
dev_features MTU MAC CTRL_VQ CTRL_MAC_ADDR ANY_LAYOUT VERSION_1 ACCESS_PLATFORM

But it should be sufficient to
use features_src & feature_dst in this case. Actually, it should work
similar as to the cpu flags, the management software should introduce
the concept of cluster which means the maximal set of common features
is calculated and provisioned during device creation to allow
migration among the nodes inside the cluster.

Yes, this is one way mgmt software may implement, but I am not sure if it's the only way. For e.g. for cpu flags, mgmt software can infer the guest cpus features in use from all qemu command line arguments and host cpu features/capability, which doesn't need to remember creation arguments and is easy to recover from failure without having to make the VM config persistent in data store. I thought it would be great if vdpa CLI design could offer the same.

One minor difference is that we have cpu model abstraction, so we can
have things like:

./qemu-system-x86_64 -cpu EPYC

Which implies the cpu features/flags where vDPA doesn't have. But
consider it's just a 64bit (or 128 in the future), it doesn't seems to
be too complex for the management to know, we probably need to start
from this and then we can try to introduce some generation/model after
it is agreed on most of the vendors.

What you refer to is the so-called named model for CPU flags. I think it's a good addition to have some generation or named model defined for vDPA. But I don't get the point for how it relates to exposing the actual value of device features? Are you saying in this case you'd rather expose the model name than the actual value of feature bits? Well, I think we can expose both in different fields when there's really such a need.
It's something like:

vdpa dev add name dev1 mgmtdev vdpasim_net device_features VDPA_NET_MODEL_1

and VDPA_NET_MODEL_1 implies some feature sets.

Not sure if this could be very useful for virtio devices, given we don't have a determined set of virtio features unlike CPU generation/model, but I think even with the features_default property we can still achieve that.

-Siwei


BTW with regard to the cpu model in mgmt software implementation, the one implemented in libvirt is a mixed "Host model" [1] with taking advantage of QEMU named model and exposing additional individual CPU features that gets close to what host CPU offers.
So creating vDPA device without "device_features" is somehow the host
model, it will have all features that is supported by the parent.

I think this implies that mgmt software should have to understand what the model name really means in terms of individual CPU features, so having feature bit value exposed will just do more help if vDPA goes the same way.
Exactly.

Thanks


Regards,
-Siwei

[1] https://qemu-project.gitlab.io/qemu/system/qemu-cpu-models.html#two-ways-to-configure-cpu-models-with-qemu-kvm


Thanks

Thanks,
-Siwei


Thanks

Thanks,
-Siwei


2) provision vDPA device with a subset of the features

# vdpa dev add name dev1 mgmtdev vdpasim_net device_features 0x300020000
# vdpa dev config show
dev1: mac 00:00:00:00:00:00 link up link_announce false mtu 1500
negotiated_features CTRL_VQ VERSION_1 ACCESS_PLATFORM

Reviewed-by: Eli Cohen <elic@xxxxxxxxxx>
Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
---
drivers/vdpa/vdpa_sim/vdpa_sim_net.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim_net.c b/drivers/vdpa/vdpa_sim/vdpa_sim_net.c
index 886449e88502..a9ba02be378b 100644
--- a/drivers/vdpa/vdpa_sim/vdpa_sim_net.c
+++ b/drivers/vdpa/vdpa_sim/vdpa_sim_net.c
@@ -254,6 +254,14 @@ static int vdpasim_net_dev_add(struct vdpa_mgmt_dev *mdev, const char *name,
dev_attr.work_fn = vdpasim_net_work;
dev_attr.buffer_size = PAGE_SIZE;

+ if (config->mask & BIT_ULL(VDPA_ATTR_DEV_FEATURES)) {
+ if (config->device_features &
+ ~dev_attr.supported_features)
+ return -EINVAL;
+ dev_attr.supported_features &=
+ config->device_features;
+ }
+
simdev = vdpasim_create(&dev_attr);
if (IS_ERR(simdev))
return PTR_ERR(simdev);
@@ -294,7 +302,8 @@ static struct vdpa_mgmt_dev mgmt_dev = {
.id_table = id_table,
.ops = &vdpasim_net_mgmtdev_ops,
.config_attr_mask = (1 << VDPA_ATTR_DEV_NET_CFG_MACADDR |
- 1 << VDPA_ATTR_DEV_NET_CFG_MTU),
+ 1 << VDPA_ATTR_DEV_NET_CFG_MTU |
+ 1 << VDPA_ATTR_DEV_FEATURES),
.max_supported_vqs = VDPASIM_NET_VQ_NUM,
.supported_features = VDPASIM_NET_FEATURES,
};