Re: [PATCH V5 0/6] ifcvf/vDPA: support query device config space through netlink

Re: [PATCH V5 0/6] ifcvf/vDPA: support query device config space through netlink

[Michael S. Tsirkin]

On Fri, Aug 12, 2022 at 06:44:54PM +0800, Zhu Lingshan wrote:
> This series allows userspace to query device config space of vDPA
> devices and the management devices through netlink,
> to get multi-queue, feature bits and etc.
> 
> This series has introduced a new netlink attr
> VDPA_ATTR_VDPA_DEV_SUPPORTED_FEATURES, this should be used to query
> features of vDPA  devices than the management device.
> 
> Please help review.

I can't merge this for this merge window.
Am I right when I say that the new thing here is patch 5/6 + new
comments?
If yes I can queue it out of the window, on top.

> Thanks!
> Zhu Lingshan
> 
> Changes rom V4:
> (1) Read MAC, MTU, MQ conditionally (Michael)
> (2) If VIRTIO_NET_F_MAC not set, don't report MAC to userspace
> (3) If VIRTIO_NET_F_MTU not set, report 1500 to userspace
> (4) Add comments to the new attr
> VDPA_ATTR_VDPA_DEV_SUPPORTED_FEATURES(Michael)
> (5) Add comments for reporting the device status as LE(Michael)
> 
> Changes from V3:
> (1)drop the fixes tags(Parva)
> (2)better commit log for patch 1/6(Michael)
> (3)assign num_queues to max_supported_vqs than max_vq_pairs(Jason)
> (4)initialize virtio pci capabilities in the probe() function.
> 
> Changes from V2:
> Add fixes tags(Parva)
> 
> Changes from V1:
> (1) Use __virito16_to_cpu(true, xxx) for the le16 casting(Jason)
> (2) Add a comment in ifcvf_get_config_size(), to explain
> why we should return the minimum value of
> sizeof(struct virtio_net_config) and the onboard
> cap size(Jason)
> (3) Introduced a new attr VDPA_ATTR_VDPA_DEV_SUPPORTED_FEATURES
> (4) Show the changes of iproute2 output before and after 5/6 patch(Jason)
> (5) Fix cast warning in vdpa_fill_stats_rec() 
> 
> Zhu Lingshan (6):
>   vDPA/ifcvf: get_config_size should return a value no greater than dev
>     implementation
>   vDPA/ifcvf: support userspace to query features and MQ of a management
>     device
>   vDPA: allow userspace to query features of a vDPA device
>   vDPA: !FEATURES_OK should not block querying device config space
>   vDPA: Conditionally read fields in virtio-net dev config space
>   fix 'cast to restricted le16' warnings in vdpa.c
> 
>  drivers/vdpa/ifcvf/ifcvf_base.c |  13 ++-
>  drivers/vdpa/ifcvf/ifcvf_base.h |   2 +
>  drivers/vdpa/ifcvf/ifcvf_main.c | 142 +++++++++++++++++---------------
>  drivers/vdpa/vdpa.c             |  82 ++++++++++++------
>  include/uapi/linux/vdpa.h       |   3 +
>  5 files changed, 149 insertions(+), 93 deletions(-)
> 
> -- 
> 2.31.1

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Re: [PATCH V5 0/6] ifcvf/vDPA: support query device config space through netlink

[Michael S. Tsirkin]

On Fri, Aug 12, 2022 at 07:14:39AM -0400, Michael S. Tsirkin wrote:
> On Fri, Aug 12, 2022 at 06:44:54PM +0800, Zhu Lingshan wrote:
> > This series allows userspace to query device config space of vDPA
> > devices and the management devices through netlink,
> > to get multi-queue, feature bits and etc.
> > 
> > This series has introduced a new netlink attr
> > VDPA_ATTR_VDPA_DEV_SUPPORTED_FEATURES, this should be used to query
> > features of vDPA  devices than the management device.
> > 
> > Please help review.
> 
> I can't merge this for this merge window.
> Am I right when I say that the new thing here is patch 5/6 + new
> comments?
> If yes I can queue it out of the window, on top.

So at this point, can you please send patches on top of the vhost
tree? I think these are just patches 3 and 5 but please confirm.


> > Thanks!
> > Zhu Lingshan
> > 
> > Changes rom V4:
> > (1) Read MAC, MTU, MQ conditionally (Michael)
> > (2) If VIRTIO_NET_F_MAC not set, don't report MAC to userspace
> > (3) If VIRTIO_NET_F_MTU not set, report 1500 to userspace
> > (4) Add comments to the new attr
> > VDPA_ATTR_VDPA_DEV_SUPPORTED_FEATURES(Michael)
> > (5) Add comments for reporting the device status as LE(Michael)
> > 
> > Changes from V3:
> > (1)drop the fixes tags(Parva)
> > (2)better commit log for patch 1/6(Michael)
> > (3)assign num_queues to max_supported_vqs than max_vq_pairs(Jason)
> > (4)initialize virtio pci capabilities in the probe() function.
> > 
> > Changes from V2:
> > Add fixes tags(Parva)
> > 
> > Changes from V1:
> > (1) Use __virito16_to_cpu(true, xxx) for the le16 casting(Jason)
> > (2) Add a comment in ifcvf_get_config_size(), to explain
> > why we should return the minimum value of
> > sizeof(struct virtio_net_config) and the onboard
> > cap size(Jason)
> > (3) Introduced a new attr VDPA_ATTR_VDPA_DEV_SUPPORTED_FEATURES
> > (4) Show the changes of iproute2 output before and after 5/6 patch(Jason)
> > (5) Fix cast warning in vdpa_fill_stats_rec() 
> > 
> > Zhu Lingshan (6):
> >   vDPA/ifcvf: get_config_size should return a value no greater than dev
> >     implementation
> >   vDPA/ifcvf: support userspace to query features and MQ of a management
> >     device
> >   vDPA: allow userspace to query features of a vDPA device
> >   vDPA: !FEATURES_OK should not block querying device config space
> >   vDPA: Conditionally read fields in virtio-net dev config space
> >   fix 'cast to restricted le16' warnings in vdpa.c
> > 
> >  drivers/vdpa/ifcvf/ifcvf_base.c |  13 ++-
> >  drivers/vdpa/ifcvf/ifcvf_base.h |   2 +
> >  drivers/vdpa/ifcvf/ifcvf_main.c | 142 +++++++++++++++++---------------
> >  drivers/vdpa/vdpa.c             |  82 ++++++++++++------
> >  include/uapi/linux/vdpa.h       |   3 +
> >  5 files changed, 149 insertions(+), 93 deletions(-)
> > 
> > -- 
> > 2.31.1

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Re: [PATCH V5 4/6] vDPA: !FEATURES_OK should not block querying device config space

[Si-Wei Liu]

Hi Michael,

I just noticed this patch got pulled to linux-next prematurely without 
getting consensus on code review, am not sure why. Hope it was just an 
oversight.

Unfortunately this introduced functionality regression to at least two 
cases so far as I see:

1. (bogus) VDPA_ATTR_DEV_NEGOTIATED_FEATURES are inadvertently exposed 
and displayed in "vdpa dev config show" before feature negotiation is 
done. Noted the corresponding features name shown in vdpa tool is called 
"negotiated_features" rather than "driver_features". I see in no way the 
intended change of the patch should break this user level expectation 
regardless of any spec requirement. Do you agree on this point?

2. There was also another implicit assumption that is broken by this 
patch. There could be a vdpa tool query of config via 
vdpa_dev_net_config_fill()->vdpa_get_config_unlocked() that races with 
the first vdpa_set_features() call from VMM e.g. QEMU. Since the 
S_FEATURES_OK blocking condition is removed, if the vdpa tool query 
occurs earlier than the first set_driver_features() call from VMM, the 
following code will treat the guest as legacy and then trigger an 
erroneous vdpa_set_features_unlocked(... , 0) call to the vdpa driver:

  374         /*
  375          * Config accesses aren't supposed to trigger before 
features are set.
  376          * If it does happen we assume a legacy guest.
  377          */
  378         if (!vdev->features_valid)
  379                 vdpa_set_features_unlocked(vdev, 0);
  380         ops->get_config(vdev, offset, buf, len);

Depending on vendor driver's implementation, L380 may either return 
invalid config data (or invalid endianness if on BE) or only config 
fields that are valid in legacy layout. What's more severe is that, vdpa 
tool query in theory shouldn't affect feature negotiation at all by 
making confusing calls to the device, but now it is possible with the 
patch. Fixing this would require more delicate work on the other paths 
involving the cf_lock reader/write semaphore.

Not sure what you plan to do next, post the fixes for both issues and 
get the community review? Or simply revert the patch in question? Let us 
know.

Thanks,
-Siwei


On 8/12/2022 3:44 AM, Zhu Lingshan wrote:
> Users may want to query the config space of a vDPA device,
> to choose a appropriate one for a certain guest. This means the
> users need to read the config space before FEATURES_OK, and
> the existence of config space contents does not depend on
> FEATURES_OK.
>
> The spec says:
> The device MUST allow reading of any device-specific configuration
> field before FEATURES_OK is set by the driver. This includes
> fields which are conditional on feature bits, as long as those
> feature bits are offered by the device.
>
> Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
> ---
>   drivers/vdpa/vdpa.c | 8 --------
>   1 file changed, 8 deletions(-)
>
> diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
> index 6eb3d972d802..bf312d9c59ab 100644
> --- a/drivers/vdpa/vdpa.c
> +++ b/drivers/vdpa/vdpa.c
> @@ -855,17 +855,9 @@ vdpa_dev_config_fill(struct vdpa_device *vdev, struct sk_buff *msg, u32 portid,
>   {
>   	u32 device_id;
>   	void *hdr;
> -	u8 status;
>   	int err;
>   
>   	down_read(&vdev->cf_lock);
> -	status = vdev->config->get_status(vdev);
> -	if (!(status & VIRTIO_CONFIG_S_FEATURES_OK)) {
> -		NL_SET_ERR_MSG_MOD(extack, "Features negotiation not completed");
> -		err = -EAGAIN;
> -		goto out;
> -	}
> -
>   	hdr = genlmsg_put(msg, portid, seq, &vdpa_nl_family, flags,
>   			  VDPA_CMD_DEV_CONFIG_GET);
>   	if (!hdr) {

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Re: [PATCH V5 4/6] vDPA: !FEATURES_OK should not block querying device config space

[Michael S. Tsirkin]

On Tue, Aug 16, 2022 at 12:41:21AM -0700, Si-Wei Liu wrote:
> Hi Michael,
> 
> I just noticed this patch got pulled to linux-next prematurely without
> getting consensus on code review, am not sure why. Hope it was just an
> oversight.
> 
> Unfortunately this introduced functionality regression to at least two cases
> so far as I see:
> 
> 1. (bogus) VDPA_ATTR_DEV_NEGOTIATED_FEATURES are inadvertently exposed and
> displayed in "vdpa dev config show" before feature negotiation is done.
> Noted the corresponding features name shown in vdpa tool is called
> "negotiated_features" rather than "driver_features". I see in no way the
> intended change of the patch should break this user level expectation
> regardless of any spec requirement. Do you agree on this point?
> 
> 2. There was also another implicit assumption that is broken by this patch.
> There could be a vdpa tool query of config via
> vdpa_dev_net_config_fill()->vdpa_get_config_unlocked() that races with the
> first vdpa_set_features() call from VMM e.g. QEMU. Since the S_FEATURES_OK
> blocking condition is removed, if the vdpa tool query occurs earlier than
> the first set_driver_features() call from VMM, the following code will treat
> the guest as legacy and then trigger an erroneous
> vdpa_set_features_unlocked(... , 0) call to the vdpa driver:
> 
>  374         /*
>  375          * Config accesses aren't supposed to trigger before features
> are set.
>  376          * If it does happen we assume a legacy guest.
>  377          */
>  378         if (!vdev->features_valid)
>  379                 vdpa_set_features_unlocked(vdev, 0);
>  380         ops->get_config(vdev, offset, buf, len);
> Depending on vendor driver's implementation, L380 may either return invalid
> config data (or invalid endianness if on BE) or only config fields that are
> valid in legacy layout. What's more severe is that, vdpa tool query in
> theory shouldn't affect feature negotiation at all by making confusing calls
> to the device, but now it is possible with the patch. Fixing this would
> require more delicate work on the other paths involving the cf_lock
> reader/write semaphore.
> 
> Not sure what you plan to do next, post the fixes for both issues and get
> the community review? Or simply revert the patch in question? Let us know.
> 
> Thanks,
> -Siwei

If we can get fixes that's good. If not I can apply a revert.
I'm on vacation next week, you guys will have the time
to figure out the best plan of action.

-- 
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Re: [PATCH V5 4/6] vDPA: !FEATURES_OK should not block querying device config space

[Michael S. Tsirkin]

On Tue, Aug 16, 2022 at 04:29:04PM +0800, Zhu, Lingshan wrote:
> 
> 
> On 8/16/2022 3:41 PM, Si-Wei Liu wrote:
> 
>     Hi Michael,
> 
>     I just noticed this patch got pulled to linux-next prematurely without
>     getting consensus on code review, am not sure why. Hope it was just an
>     oversight.
> 
>     Unfortunately this introduced functionality regression to at least two
>     cases so far as I see:
> 
>     1. (bogus) VDPA_ATTR_DEV_NEGOTIATED_FEATURES are inadvertently exposed and
>     displayed in "vdpa dev config show" before feature negotiation is done.
>     Noted the corresponding features name shown in vdpa tool is called
>     "negotiated_features" rather than "driver_features". I see in no way the
>     intended change of the patch should break this user level expectation
>     regardless of any spec requirement. Do you agree on this point?
> 
> I will post a patch for iptour2, doing:
> 1) if iprout2 does not get driver_features from the kernel, then don't show
> negotiated features in the command output
> 2) process and decoding the device features.
> 
> 
>     2. There was also another implicit assumption that is broken by this patch.
>     There could be a vdpa tool query of config via vdpa_dev_net_config_fill()->
>     vdpa_get_config_unlocked() that races with the first vdpa_set_features()
>     call from VMM e.g. QEMU. Since the S_FEATURES_OK blocking condition is
>     removed, if the vdpa tool query occurs earlier than the first
>     set_driver_features() call from VMM, the following code will treat the
>     guest as legacy and then trigger an erroneous vdpa_set_features_unlocked
>     (... , 0) call to the vdpa driver:
> 
>      374         /*
>      375          * Config accesses aren't supposed to trigger before features
>     are set.
>      376          * If it does happen we assume a legacy guest.
>      377          */
>      378         if (!vdev->features_valid)
>      379                 vdpa_set_features_unlocked(vdev, 0);
>      380         ops->get_config(vdev, offset, buf, len);
> 
>     Depending on vendor driver's implementation, L380 may either return invalid
>     config data (or invalid endianness if on BE) or only config fields that are
>     valid in legacy layout. What's more severe is that, vdpa tool query in
>     theory shouldn't affect feature negotiation at all by making confusing
>     calls to the device, but now it is possible with the patch. Fixing this
>     would require more delicate work on the other paths involving the cf_lock
>     reader/write semaphore.
> 
>     Not sure what you plan to do next, post the fixes for both issues and get
>     the community review? Or simply revert the patch in question? Let us know.
> 
> The spec says:
> The device MUST allow reading of any device-specific configuration field before
> FEATURES_OK is set by
> the driver. This includes fields which are conditional on feature bits, as long
> as those feature bits are offered
> by the device.
> 
> so whether FEATURES_OK should not block reading the device config space. 
> vdpa_get_config_unlocked() will read the features, I don't know why it has a
> comment:
>         /*
>          * Config accesses aren't supposed to trigger before features are set.
>          * If it does happen we assume a legacy guest.
>          */
> 
> This conflicts with the spec.

Yea well. On the other hand the spec also calls for features to be
used to detect legacy versus modern driver.
This part of the spec needs work generally.


> vdpa_get_config_unlocked() checks vdev->features_valid, if not valid, it will
> set the drivers_features 0, I think this intends to prevent reading random
> driver_features. This function does not hold any locks, and didn't change
> anything.
> 
> So what is the race?
> 
> Thanks
> 
> 
> 
>     Thanks,
>     -Siwei
> 
> 
>     On 8/12/2022 3:44 AM, Zhu Lingshan wrote:
> 
>         Users may want to query the config space of a vDPA device,
>         to choose a appropriate one for a certain guest. This means the
>         users need to read the config space before FEATURES_OK, and
>         the existence of config space contents does not depend on
>         FEATURES_OK.
> 
>         The spec says:
>         The device MUST allow reading of any device-specific configuration
>         field before FEATURES_OK is set by the driver. This includes
>         fields which are conditional on feature bits, as long as those
>         feature bits are offered by the device.
> 
>         Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
>         ---
>           drivers/vdpa/vdpa.c | 8 --------
>           1 file changed, 8 deletions(-)
> 
>         diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
>         index 6eb3d972d802..bf312d9c59ab 100644
>         --- a/drivers/vdpa/vdpa.c
>         +++ b/drivers/vdpa/vdpa.c
>         @@ -855,17 +855,9 @@ vdpa_dev_config_fill(struct vdpa_device *vdev,
>         struct sk_buff *msg, u32 portid,
>           {
>               u32 device_id;
>               void *hdr;
>         -    u8 status;
>               int err;
>                 down_read(&vdev->cf_lock);
>         -    status = vdev->config->get_status(vdev);
>         -    if (!(status & VIRTIO_CONFIG_S_FEATURES_OK)) {
>         -        NL_SET_ERR_MSG_MOD(extack, "Features negotiation not
>         completed");
>         -        err = -EAGAIN;
>         -        goto out;
>         -    }
>         -
>               hdr = genlmsg_put(msg, portid, seq, &vdpa_nl_family, flags,
>                         VDPA_CMD_DEV_CONFIG_GET);
>               if (!hdr) {
> 
> 
> 
> 

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Re: [PATCH V5 4/6] vDPA: !FEATURES_OK should not block querying device config space

[Michael S. Tsirkin]

On Tue, Aug 16, 2022 at 12:41:21AM -0700, Si-Wei Liu wrote:
> Hi Michael,
> 
> I just noticed this patch got pulled to linux-next prematurely without
> getting consensus on code review, am not sure why. Hope it was just an
> oversight.
> 
> Unfortunately this introduced functionality regression to at least two cases
> so far as I see:
> 
> 1. (bogus) VDPA_ATTR_DEV_NEGOTIATED_FEATURES are inadvertently exposed and
> displayed in "vdpa dev config show" before feature negotiation is done.
> Noted the corresponding features name shown in vdpa tool is called
> "negotiated_features" rather than "driver_features". I see in no way the
> intended change of the patch should break this user level expectation
> regardless of any spec requirement. Do you agree on this point?
> 
> 2. There was also another implicit assumption that is broken by this patch.
> There could be a vdpa tool query of config via
> vdpa_dev_net_config_fill()->vdpa_get_config_unlocked() that races with the
> first vdpa_set_features() call from VMM e.g. QEMU. Since the S_FEATURES_OK
> blocking condition is removed, if the vdpa tool query occurs earlier than
> the first set_driver_features() call from VMM, the following code will treat
> the guest as legacy and then trigger an erroneous
> vdpa_set_features_unlocked(... , 0) call to the vdpa driver:
> 
>  374         /*
>  375          * Config accesses aren't supposed to trigger before features
> are set.
>  376          * If it does happen we assume a legacy guest.
>  377          */
>  378         if (!vdev->features_valid)
>  379                 vdpa_set_features_unlocked(vdev, 0);
>  380         ops->get_config(vdev, offset, buf, len);
> 
> Depending on vendor driver's implementation, L380 may either return invalid
> config data (or invalid endianness if on BE) or only config fields that are
> valid in legacy layout. What's more severe is that, vdpa tool query in
> theory shouldn't affect feature negotiation at all by making confusing calls
> to the device, but now it is possible with the patch. Fixing this would
> require more delicate work on the other paths involving the cf_lock
> reader/write semaphore.
> 
> Not sure what you plan to do next, post the fixes for both issues and get
> the community review? Or simply revert the patch in question? Let us know.
> 
> Thanks,
> -Siwei
> 

I'm not sure who you are asking. I didn't realize this is so
controversial. If you feel it should be reverted I suggest
you post a revert patch with a detailed motivation and this
will get the discussion going.
It will also help if you stress whether you describe theoretical
issues or something observed in practice above
discussion does not make this clear.

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Re: [PATCH V5 4/6] vDPA: !FEATURES_OK should not block querying device config space

[Si-Wei Liu]

On 8/16/2022 2:13 PM, Michael S. Tsirkin wrote:
> On Tue, Aug 16, 2022 at 12:41:21AM -0700, Si-Wei Liu wrote:
>> Hi Michael,
>>
>> I just noticed this patch got pulled to linux-next prematurely without
>> getting consensus on code review, am not sure why. Hope it was just an
>> oversight.
>>
>> Unfortunately this introduced functionality regression to at least two cases
>> so far as I see:
>>
>> 1. (bogus) VDPA_ATTR_DEV_NEGOTIATED_FEATURES are inadvertently exposed and
>> displayed in "vdpa dev config show" before feature negotiation is done.
>> Noted the corresponding features name shown in vdpa tool is called
>> "negotiated_features" rather than "driver_features". I see in no way the
>> intended change of the patch should break this user level expectation
>> regardless of any spec requirement. Do you agree on this point?
>>
>> 2. There was also another implicit assumption that is broken by this patch.
>> There could be a vdpa tool query of config via
>> vdpa_dev_net_config_fill()->vdpa_get_config_unlocked() that races with the
>> first vdpa_set_features() call from VMM e.g. QEMU. Since the S_FEATURES_OK
>> blocking condition is removed, if the vdpa tool query occurs earlier than
>> the first set_driver_features() call from VMM, the following code will treat
>> the guest as legacy and then trigger an erroneous
>> vdpa_set_features_unlocked(... , 0) call to the vdpa driver:
>>
>>   374         /*
>>   375          * Config accesses aren't supposed to trigger before features
>> are set.
>>   376          * If it does happen we assume a legacy guest.
>>   377          */
>>   378         if (!vdev->features_valid)
>>   379                 vdpa_set_features_unlocked(vdev, 0);
>>   380         ops->get_config(vdev, offset, buf, len);
>>
>> Depending on vendor driver's implementation, L380 may either return invalid
>> config data (or invalid endianness if on BE) or only config fields that are
>> valid in legacy layout. What's more severe is that, vdpa tool query in
>> theory shouldn't affect feature negotiation at all by making confusing calls
>> to the device, but now it is possible with the patch. Fixing this would
>> require more delicate work on the other paths involving the cf_lock
>> reader/write semaphore.
>>
>> Not sure what you plan to do next, post the fixes for both issues and get
>> the community review? Or simply revert the patch in question? Let us know.
>>
>> Thanks,
>> -Siwei
>>
> I'm not sure who you are asking. I didn't realize this is so
> controversial. If you feel it should be reverted I suggest
> you post a revert patch with a detailed motivation and this
> will get the discussion going.
Leave it around then, until the next person shout out aloud. I don't 
mind taking personal time to help, though my impression of the past 
conversation is that this is less productive way of cooperation and 
collaboration.

Please safely ignore me from now on.

-Siwei


> It will also help if you stress whether you describe theoretical
> issues or something observed in practice above
> discussion does not make this clear.
>

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Re: [PATCH V5 4/6] vDPA: !FEATURES_OK should not block querying device config space

[Si-Wei Liu]

[-- Attachment #1.1: Type: text/plain, Size: 6173 bytes --]



On 8/16/2022 1:29 AM, Zhu, Lingshan wrote:
>
>
> On 8/16/2022 3:41 PM, Si-Wei Liu wrote:
>> Hi Michael,
>>
>> I just noticed this patch got pulled to linux-next prematurely 
>> without getting consensus on code review, am not sure why. Hope it 
>> was just an oversight.
>>
>> Unfortunately this introduced functionality regression to at least 
>> two cases so far as I see:
>>
>> 1. (bogus) VDPA_ATTR_DEV_NEGOTIATED_FEATURES are inadvertently 
>> exposed and displayed in "vdpa dev config show" before feature 
>> negotiation is done. Noted the corresponding features name shown in 
>> vdpa tool is called "negotiated_features" rather than 
>> "driver_features". I see in no way the intended change of the patch 
>> should break this user level expectation regardless of any spec 
>> requirement. Do you agree on this point?
> I will post a patch for iptour2, doing:
> 1) if iprout2 does not get driver_features from the kernel, then don't 
> show negotiated features in the command output
This won't work as the vdpa userspace tool won't know *when* features 
are negotiated. There's no guarantee in the kernel to assume 0 will be 
returned from vendor driver during negotiation. On the other hand, with 
the supposed change, userspace can't tell if there's really none of 
features negotiated, or the feature negotiation is over. Before the 
change the userspace either gets all the attributes when feature 
negotiation is over, or it gets nothing when it's ongoing, so there was 
a distinction.This expectation of what "negotiated_features" represents 
is established from day one, I see no reason the intended kernel change 
to show other attributes should break userspace behavior and user's 
expectation.
> 2) process and decoding the device features.
>>
>> 2. There was also another implicit assumption that is broken by this 
>> patch. There could be a vdpa tool query of config via 
>> vdpa_dev_net_config_fill()->vdpa_get_config_unlocked() that races 
>> with the first vdpa_set_features() call from VMM e.g. QEMU. Since the 
>> S_FEATURES_OK blocking condition is removed, if the vdpa tool query 
>> occurs earlier than the first set_driver_features() call from VMM, 
>> the following code will treat the guest as legacy and then trigger an 
>> erroneous vdpa_set_features_unlocked(... , 0) call to the vdpa driver:
>>
>>  374         /*
>>  375          * Config accesses aren't supposed to trigger before 
>> features are set.
>>  376          * If it does happen we assume a legacy guest.
>>  377          */
>>  378         if (!vdev->features_valid)
>>  379                 vdpa_set_features_unlocked(vdev, 0);
>>  380         ops->get_config(vdev, offset, buf, len);
>>
>> Depending on vendor driver's implementation, L380 may either return 
>> invalid config data (or invalid endianness if on BE) or only config 
>> fields that are valid in legacy layout. What's more severe is that, 
>> vdpa tool query in theory shouldn't affect feature negotiation at all 
>> by making confusing calls to the device, but now it is possible with 
>> the patch. Fixing this would require more delicate work on the other 
>> paths involving the cf_lock reader/write semaphore.
>>
>> Not sure what you plan to do next, post the fixes for both issues and 
>> get the community review? Or simply revert the patch in question? Let 
>> us know.
> The spec says:
> The device MUST allow reading of any device-specific configuration 
> field before FEATURES_OK is set by
> the driver. This includes fields which are conditional on feature 
> bits, as long as those feature bits are offered
> by the device.
>
> so whether FEATURES_OK should not block reading the device config 
> space. vdpa_get_config_unlocked() will read the features, I don't know 
> why it has a comment:
>         /*
>          * Config accesses aren't supposed to trigger before features 
> are set.
>          * If it does happen we assume a legacy guest.
>          */
>
> This conflicts with the spec.
>
> vdpa_get_config_unlocked() checks vdev->features_valid, if not valid, 
> it will set the drivers_features 0, I think this intends to prevent 
> reading random driver_features. This function does not hold any locks, 
> and didn't change anything.
>
> So what is the race?
You'll see the race if you keep 'vdpa dev config show ...' running in a 
tight loop while launching a VM with the vDPA device under query.

-Siwei


>
> Thanks
>
>>
>> Thanks,
>> -Siwei
>>
>>
>> On 8/12/2022 3:44 AM, Zhu Lingshan wrote:
>>> Users may want to query the config space of a vDPA device,
>>> to choose a appropriate one for a certain guest. This means the
>>> users need to read the config space before FEATURES_OK, and
>>> the existence of config space contents does not depend on
>>> FEATURES_OK.
>>>
>>> The spec says:
>>> The device MUST allow reading of any device-specific configuration
>>> field before FEATURES_OK is set by the driver. This includes
>>> fields which are conditional on feature bits, as long as those
>>> feature bits are offered by the device.
>>>
>>> Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
>>> ---
>>>   drivers/vdpa/vdpa.c | 8 --------
>>>   1 file changed, 8 deletions(-)
>>>
>>> diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
>>> index 6eb3d972d802..bf312d9c59ab 100644
>>> --- a/drivers/vdpa/vdpa.c
>>> +++ b/drivers/vdpa/vdpa.c
>>> @@ -855,17 +855,9 @@ vdpa_dev_config_fill(struct vdpa_device *vdev, 
>>> struct sk_buff *msg, u32 portid,
>>>   {
>>>       u32 device_id;
>>>       void *hdr;
>>> -    u8 status;
>>>       int err;
>>>         down_read(&vdev->cf_lock);
>>> -    status = vdev->config->get_status(vdev);
>>> -    if (!(status & VIRTIO_CONFIG_S_FEATURES_OK)) {
>>> -        NL_SET_ERR_MSG_MOD(extack, "Features negotiation not 
>>> completed");
>>> -        err = -EAGAIN;
>>> -        goto out;
>>> -    }
>>> -
>>>       hdr = genlmsg_put(msg, portid, seq, &vdpa_nl_family, flags,
>>>                 VDPA_CMD_DEV_CONFIG_GET);
>>>       if (!hdr) {
>>
>

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Re: [PATCH V5 4/6] vDPA: !FEATURES_OK should not block querying device config space

[Michael S. Tsirkin]

On Wed, Aug 17, 2022 at 10:11:36AM +0800, Zhu, Lingshan wrote:
> 
> 
> On 8/17/2022 6:48 AM, Si-Wei Liu wrote:
> 
> 
> 
>     On 8/16/2022 1:29 AM, Zhu, Lingshan wrote:
> 
> 
> 
>         On 8/16/2022 3:41 PM, Si-Wei Liu wrote:
> 
>             Hi Michael,
> 
>             I just noticed this patch got pulled to linux-next prematurely
>             without getting consensus on code review, am not sure why. Hope it
>             was just an oversight.
> 
>             Unfortunately this introduced functionality regression to at least
>             two cases so far as I see:
> 
>             1. (bogus) VDPA_ATTR_DEV_NEGOTIATED_FEATURES are inadvertently
>             exposed and displayed in "vdpa dev config show" before feature
>             negotiation is done. Noted the corresponding features name shown in
>             vdpa tool is called "negotiated_features" rather than
>             "driver_features". I see in no way the intended change of the patch
>             should break this user level expectation regardless of any spec
>             requirement. Do you agree on this point?
> 
>         I will post a patch for iptour2, doing:
>         1) if iprout2 does not get driver_features from the kernel, then don't
>         show negotiated features in the command output
> 
>     This won't work as the vdpa userspace tool won't know *when* features are
>     negotiated. There's no guarantee in the kernel to assume 0 will be returned
>     from vendor driver during negotiation. On the other hand, with the supposed
>     change, userspace can't tell if there's really none of features negotiated,
>     or the feature negotiation is over. Before the change the userspace either
>     gets all the attributes when feature negotiation is over, or it gets
>     nothing when it's ongoing, so there was a distinction.This expectation of
>     what "negotiated_features" represents is established from day one, I see no
>     reason the intended kernel change to show other attributes should break
>     userspace behavior and user's expectation.
> 
> User space can only read valid *driver_features* after the features negotiation
> is done, *device_features* does not require the negotiation.
> 
> If you want to prevent random values read from driver_features, here I propose
> a fix: only read driver_features when the negotiation is done, this means to
> check (status & VIRTIO_CONFIG_S_FEATURES_OK) before reading the
> driver_features.
> Sounds good?
> 
> @MST, if this is OK, I can include this change in my next version patch series.
> 
> Thanks,
> Zhu Lingshan

Sorry I don't get it. Is there going to be a new version? Do you want me
to revert this one and then apply a new one? It's ok if yes.


>         2) process and decoding the device features.
> 
> 
>             2. There was also another implicit assumption that is broken by
>             this patch. There could be a vdpa tool query of config via
>             vdpa_dev_net_config_fill()->vdpa_get_config_unlocked() that races
>             with the first vdpa_set_features() call from VMM e.g. QEMU. Since
>             the S_FEATURES_OK blocking condition is removed, if the vdpa tool
>             query occurs earlier than the first set_driver_features() call from
>             VMM, the following code will treat the guest as legacy and then
>             trigger an erroneous vdpa_set_features_unlocked(... , 0) call to
>             the vdpa driver:
> 
>              374         /*
>              375          * Config accesses aren't supposed to trigger before
>             features are set.
>              376          * If it does happen we assume a legacy guest.
>              377          */
>              378         if (!vdev->features_valid)
>              379                 vdpa_set_features_unlocked(vdev, 0);
>              380         ops->get_config(vdev, offset, buf, len);
> 
>             Depending on vendor driver's implementation, L380 may either return
>             invalid config data (or invalid endianness if on BE) or only config
>             fields that are valid in legacy layout. What's more severe is that,
>             vdpa tool query in theory shouldn't affect feature negotiation at
>             all by making confusing calls to the device, but now it is possible
>             with the patch. Fixing this would require more delicate work on the
>             other paths involving the cf_lock reader/write semaphore.
> 
>             Not sure what you plan to do next, post the fixes for both issues
>             and get the community review? Or simply revert the patch in
>             question? Let us know.
> 
>         The spec says:
>         The device MUST allow reading of any device-specific configuration
>         field before FEATURES_OK is set by
>         the driver. This includes fields which are conditional on feature bits,
>         as long as those feature bits are offered
>         by the device.
> 
>         so whether FEATURES_OK should not block reading the device config
>         space. vdpa_get_config_unlocked() will read the features, I don't know
>         why it has a comment:
>                 /*
>                  * Config accesses aren't supposed to trigger before features
>         are set.
>                  * If it does happen we assume a legacy guest.
>                  */
> 
>         This conflicts with the spec.
> 
>         vdpa_get_config_unlocked() checks vdev->features_valid, if not valid,
>         it will set the drivers_features 0, I think this intends to prevent
>         reading random driver_features. This function does not hold any locks,
>         and didn't change anything.
> 
>         So what is the race?
>    
>     You'll see the race if you keep 'vdpa dev config show ...' running in a
>     tight loop while launching a VM with the vDPA device under query.
> 
>     -Siwei
> 
> 
> 
>        
>         Thanks
> 
>        
> 
>             Thanks,
>             -Siwei
> 
> 
>             On 8/12/2022 3:44 AM, Zhu Lingshan wrote:
> 
>                 Users may want to query the config space of a vDPA device,
>                 to choose a appropriate one for a certain guest. This means the
>                 users need to read the config space before FEATURES_OK, and
>                 the existence of config space contents does not depend on
>                 FEATURES_OK.
> 
>                 The spec says:
>                 The device MUST allow reading of any device-specific
>                 configuration
>                 field before FEATURES_OK is set by the driver. This includes
>                 fields which are conditional on feature bits, as long as those
>                 feature bits are offered by the device.
> 
>                 Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
>                 ---
>                   drivers/vdpa/vdpa.c | 8 --------
>                   1 file changed, 8 deletions(-)
> 
>                 diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
>                 index 6eb3d972d802..bf312d9c59ab 100644
>                 --- a/drivers/vdpa/vdpa.c
>                 +++ b/drivers/vdpa/vdpa.c
>                 @@ -855,17 +855,9 @@ vdpa_dev_config_fill(struct vdpa_device
>                 *vdev, struct sk_buff *msg, u32 portid,
>                   {
>                       u32 device_id;
>                       void *hdr;
>                 -    u8 status;
>                       int err;
>                         down_read(&vdev->cf_lock);
>                 -    status = vdev->config->get_status(vdev);
>                 -    if (!(status & VIRTIO_CONFIG_S_FEATURES_OK)) {
>                 -        NL_SET_ERR_MSG_MOD(extack, "Features negotiation not
>                 completed");
>                 -        err = -EAGAIN;
>                 -        goto out;
>                 -    }
>                 -
>                       hdr = genlmsg_put(msg, portid, seq, &vdpa_nl_family,
>                 flags,
>                                 VDPA_CMD_DEV_CONFIG_GET);
>                       if (!hdr) {
> 
> 
> 
> 
> 
> 
> 
> 

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Re: [PATCH V5 4/6] vDPA: !FEATURES_OK should not block querying device config space

[Si-Wei Liu]

[-- Attachment #1.1: Type: text/plain, Size: 11917 bytes --]



On 8/16/2022 11:23 PM, Zhu, Lingshan wrote:
>
>
> On 8/17/2022 2:14 PM, Michael S. Tsirkin wrote:
>> On Wed, Aug 17, 2022 at 10:11:36AM +0800, Zhu, Lingshan wrote:
>>>
>>> On 8/17/2022 6:48 AM, Si-Wei Liu wrote:
>>>
>>>
>>>
>>>      On 8/16/2022 1:29 AM, Zhu, Lingshan wrote:
>>>
>>>
>>>
>>>          On 8/16/2022 3:41 PM, Si-Wei Liu wrote:
>>>
>>>              Hi Michael,
>>>
>>>              I just noticed this patch got pulled to linux-next 
>>> prematurely
>>>              without getting consensus on code review, am not sure 
>>> why. Hope it
>>>              was just an oversight.
>>>
>>>              Unfortunately this introduced functionality regression 
>>> to at least
>>>              two cases so far as I see:
>>>
>>>              1. (bogus) VDPA_ATTR_DEV_NEGOTIATED_FEATURES are 
>>> inadvertently
>>>              exposed and displayed in "vdpa dev config show" before 
>>> feature
>>>              negotiation is done. Noted the corresponding features 
>>> name shown in
>>>              vdpa tool is called "negotiated_features" rather than
>>>              "driver_features". I see in no way the intended change 
>>> of the patch
>>>              should break this user level expectation regardless of 
>>> any spec
>>>              requirement. Do you agree on this point?
>>>
>>>          I will post a patch for iptour2, doing:
>>>          1) if iprout2 does not get driver_features from the kernel, 
>>> then don't
>>>          show negotiated features in the command output
>>>
>>>      This won't work as the vdpa userspace tool won't know *when* 
>>> features are
>>>      negotiated. There's no guarantee in the kernel to assume 0 will 
>>> be returned
>>>      from vendor driver during negotiation. On the other hand, with 
>>> the supposed
>>>      change, userspace can't tell if there's really none of features 
>>> negotiated,
>>>      or the feature negotiation is over. Before the change the 
>>> userspace either
>>>      gets all the attributes when feature negotiation is over, or it 
>>> gets
>>>      nothing when it's ongoing, so there was a distinction.This 
>>> expectation of
>>>      what "negotiated_features" represents is established from day 
>>> one, I see no
>>>      reason the intended kernel change to show other attributes 
>>> should break
>>>      userspace behavior and user's expectation.
>>>
>>> User space can only read valid *driver_features* after the features 
>>> negotiation
>>> is done, *device_features* does not require the negotiation.
>>>
>>> If you want to prevent random values read from driver_features, here 
>>> I propose
>>> a fix: only read driver_features when the negotiation is done, this 
>>> means to
>>> check (status & VIRTIO_CONFIG_S_FEATURES_OK) before reading the
>>> driver_features.
>>> Sounds good?
>>>
>>> @MST, if this is OK, I can include this change in my next version 
>>> patch series.
>>>
>>> Thanks,
>>> Zhu Lingshan
>> Sorry I don't get it. Is there going to be a new version? Do you want me
>> to revert this one and then apply a new one? It's ok if yes.
> Not a new version, it is a new patch, though I still didn't get the 
> race condition, but I believe it
> is reasonable to block reading the *driver_features* before FEATURES_OK.
>
> So, I added code to check whether _FEATURES_OK is set:
>
>  861         /* only read driver features after the feature 
> negotiation is done */
>  862         status = vdev->config->get_status(vdev);
>  863         if (status & VIRTIO_CONFIG_S_FEATURES_OK) {
>  864                 features_driver = 
> vdev->config->get_driver_features(vdev);
>  865                 if (nla_put_u64_64bit(msg, 
> VDPA_ATTR_DEV_NEGOTIATED_FEATURES, features_driver,
>  866                                       VDPA_ATTR_PAD))
>  867                 return -EMSGSIZE;
>  868         }
>
> If this solution looks good, I will add this patch in my V2 series.
This solves the 1st issue in my report, but without a fix for the 2nd 
issue ( vdpa_dev_net_config_fill and vdpa_set_features race) addressed I 
don't think it covers all incurred regressions.

I've replied the way to reproduce the race. For me it's very obvious to 
see in my setup.
>
> > So what is the race?
> You'll see the race if you keep 'vdpa dev config show ...' running in 
> a tight loop while launching a VM with the vDPA device under query.
>
-Siwei

>
> Thanks
> Zhu Lingshan
>
>>
>>
>>>          2) process and decoding the device features.
>>>
>>>
>>>              2. There was also another implicit assumption that is 
>>> broken by
>>>              this patch. There could be a vdpa tool query of config via
>>> vdpa_dev_net_config_fill()->vdpa_get_config_unlocked() that races
>>>              with the first vdpa_set_features() call from VMM e.g. 
>>> QEMU. Since
>>>              the S_FEATURES_OK blocking condition is removed, if the 
>>> vdpa tool
>>>              query occurs earlier than the first 
>>> set_driver_features() call from
>>>              VMM, the following code will treat the guest as legacy 
>>> and then
>>>              trigger an erroneous vdpa_set_features_unlocked(... , 
>>> 0) call to
>>>              the vdpa driver:
>>>
>>>               374         /*
>>>               375          * Config accesses aren't supposed to 
>>> trigger before
>>>              features are set.
>>>               376          * If it does happen we assume a legacy 
>>> guest.
>>>               377          */
>>>               378         if (!vdev->features_valid)
>>>               379 vdpa_set_features_unlocked(vdev, 0);
>>>               380         ops->get_config(vdev, offset, buf, len);
>>>
>>>              Depending on vendor driver's implementation, L380 may 
>>> either return
>>>              invalid config data (or invalid endianness if on BE) or 
>>> only config
>>>              fields that are valid in legacy layout. What's more 
>>> severe is that,
>>>              vdpa tool query in theory shouldn't affect feature 
>>> negotiation at
>>>              all by making confusing calls to the device, but now it 
>>> is possible
>>>              with the patch. Fixing this would require more delicate 
>>> work on the
>>>              other paths involving the cf_lock reader/write semaphore.
>>>
>>>              Not sure what you plan to do next, post the fixes for 
>>> both issues
>>>              and get the community review? Or simply revert the 
>>> patch in
>>>              question? Let us know.
>>>
>>>          The spec says:
>>>          The device MUST allow reading of any device-specific 
>>> configuration
>>>          field before FEATURES_OK is set by
>>>          the driver. This includes fields which are conditional on 
>>> feature bits,
>>>          as long as those feature bits are offered
>>>          by the device.
>>>
>>>          so whether FEATURES_OK should not block reading the device 
>>> config
>>>          space. vdpa_get_config_unlocked() will read the features, I 
>>> don't know
>>>          why it has a comment:
>>>                  /*
>>>                   * Config accesses aren't supposed to trigger 
>>> before features
>>>          are set.
>>>                   * If it does happen we assume a legacy guest.
>>>                   */
>>>
>>>          This conflicts with the spec.
>>>
>>>          vdpa_get_config_unlocked() checks vdev->features_valid, if 
>>> not valid,
>>>          it will set the drivers_features 0, I think this intends to 
>>> prevent
>>>          reading random driver_features. This function does not hold 
>>> any locks,
>>>          and didn't change anything.
>>>
>>>          So what is the race?
>>>          You'll see the race if you keep 'vdpa dev config show ...' 
>>> running in a
>>>      tight loop while launching a VM with the vDPA device under query.
>>>
>>>      -Siwei
>>>
>>>
>>>
>>>                  Thanks
>>>
>>>
>>>              Thanks,
>>>              -Siwei
>>>
>>>
>>>              On 8/12/2022 3:44 AM, Zhu Lingshan wrote:
>>>
>>>                  Users may want to query the config space of a vDPA 
>>> device,
>>>                  to choose a appropriate one for a certain guest. 
>>> This means the
>>>                  users need to read the config space before 
>>> FEATURES_OK, and
>>>                  the existence of config space contents does not 
>>> depend on
>>>                  FEATURES_OK.
>>>
>>>                  The spec says:
>>>                  The device MUST allow reading of any device-specific
>>>                  configuration
>>>                  field before FEATURES_OK is set by the driver. This 
>>> includes
>>>                  fields which are conditional on feature bits, as 
>>> long as those
>>>                  feature bits are offered by the device.
>>>
>>>                  Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
>>>                  ---
>>>                    drivers/vdpa/vdpa.c | 8 --------
>>>                    1 file changed, 8 deletions(-)
>>>
>>>                  diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
>>>                  index 6eb3d972d802..bf312d9c59ab 100644
>>>                  --- a/drivers/vdpa/vdpa.c
>>>                  +++ b/drivers/vdpa/vdpa.c
>>>                  @@ -855,17 +855,9 @@ vdpa_dev_config_fill(struct 
>>> vdpa_device
>>>                  *vdev, struct sk_buff *msg, u32 portid,
>>>                    {
>>>                        u32 device_id;
>>>                        void *hdr;
>>>                  -    u8 status;
>>>                        int err;
>>>                          down_read(&vdev->cf_lock);
>>>                  -    status = vdev->config->get_status(vdev);
>>>                  -    if (!(status & VIRTIO_CONFIG_S_FEATURES_OK)) {
>>>                  -        NL_SET_ERR_MSG_MOD(extack, "Features 
>>> negotiation not
>>>                  completed");
>>>                  -        err = -EAGAIN;
>>>                  -        goto out;
>>>                  -    }
>>>                  -
>>>                        hdr = genlmsg_put(msg, portid, seq, 
>>> &vdpa_nl_family,
>>>                  flags,
>>>                                  VDPA_CMD_DEV_CONFIG_GET);
>>>                        if (!hdr) {
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>

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Re: [PATCH V5 4/6] vDPA: !FEATURES_OK should not block querying device config space

[Si-Wei Liu]

[-- Attachment #1.1: Type: text/plain, Size: 13164 bytes --]



On 8/17/2022 11:52 PM, Zhu, Lingshan wrote:
>
>
> On 8/18/2022 2:48 AM, Si-Wei Liu wrote:
>>
>>
>> On 8/16/2022 11:23 PM, Zhu, Lingshan wrote:
>>>
>>>
>>> On 8/17/2022 2:14 PM, Michael S. Tsirkin wrote:
>>>> On Wed, Aug 17, 2022 at 10:11:36AM +0800, Zhu, Lingshan wrote:
>>>>>
>>>>> On 8/17/2022 6:48 AM, Si-Wei Liu wrote:
>>>>>
>>>>>
>>>>>
>>>>>      On 8/16/2022 1:29 AM, Zhu, Lingshan wrote:
>>>>>
>>>>>
>>>>>
>>>>>          On 8/16/2022 3:41 PM, Si-Wei Liu wrote:
>>>>>
>>>>>              Hi Michael,
>>>>>
>>>>>              I just noticed this patch got pulled to linux-next 
>>>>> prematurely
>>>>>              without getting consensus on code review, am not sure 
>>>>> why. Hope it
>>>>>              was just an oversight.
>>>>>
>>>>>              Unfortunately this introduced functionality 
>>>>> regression to at least
>>>>>              two cases so far as I see:
>>>>>
>>>>>              1. (bogus) VDPA_ATTR_DEV_NEGOTIATED_FEATURES are 
>>>>> inadvertently
>>>>>              exposed and displayed in "vdpa dev config show" 
>>>>> before feature
>>>>>              negotiation is done. Noted the corresponding features 
>>>>> name shown in
>>>>>              vdpa tool is called "negotiated_features" rather than
>>>>>              "driver_features". I see in no way the intended 
>>>>> change of the patch
>>>>>              should break this user level expectation regardless 
>>>>> of any spec
>>>>>              requirement. Do you agree on this point?
>>>>>
>>>>>          I will post a patch for iptour2, doing:
>>>>>          1) if iprout2 does not get driver_features from the 
>>>>> kernel, then don't
>>>>>          show negotiated features in the command output
>>>>>
>>>>>      This won't work as the vdpa userspace tool won't know *when* 
>>>>> features are
>>>>>      negotiated. There's no guarantee in the kernel to assume 0 
>>>>> will be returned
>>>>>      from vendor driver during negotiation. On the other hand, 
>>>>> with the supposed
>>>>>      change, userspace can't tell if there's really none of 
>>>>> features negotiated,
>>>>>      or the feature negotiation is over. Before the change the 
>>>>> userspace either
>>>>>      gets all the attributes when feature negotiation is over, or 
>>>>> it gets
>>>>>      nothing when it's ongoing, so there was a distinction.This 
>>>>> expectation of
>>>>>      what "negotiated_features" represents is established from day 
>>>>> one, I see no
>>>>>      reason the intended kernel change to show other attributes 
>>>>> should break
>>>>>      userspace behavior and user's expectation.
>>>>>
>>>>> User space can only read valid *driver_features* after the 
>>>>> features negotiation
>>>>> is done, *device_features* does not require the negotiation.
>>>>>
>>>>> If you want to prevent random values read from driver_features, 
>>>>> here I propose
>>>>> a fix: only read driver_features when the negotiation is done, 
>>>>> this means to
>>>>> check (status & VIRTIO_CONFIG_S_FEATURES_OK) before reading the
>>>>> driver_features.
>>>>> Sounds good?
>>>>>
>>>>> @MST, if this is OK, I can include this change in my next version 
>>>>> patch series.
>>>>>
>>>>> Thanks,
>>>>> Zhu Lingshan
>>>> Sorry I don't get it. Is there going to be a new version? Do you 
>>>> want me
>>>> to revert this one and then apply a new one? It's ok if yes.
>>> Not a new version, it is a new patch, though I still didn't get the 
>>> race condition, but I believe it
>>> is reasonable to block reading the *driver_features* before 
>>> FEATURES_OK.
>>>
>>> So, I added code to check whether _FEATURES_OK is set:
>>>
>>>  861         /* only read driver features after the feature 
>>> negotiation is done */
>>>  862         status = vdev->config->get_status(vdev);
>>>  863         if (status & VIRTIO_CONFIG_S_FEATURES_OK) {
>>>  864                 features_driver = 
>>> vdev->config->get_driver_features(vdev);
>>>  865                 if (nla_put_u64_64bit(msg, 
>>> VDPA_ATTR_DEV_NEGOTIATED_FEATURES, features_driver,
>>>  866                                       VDPA_ATTR_PAD))
>>>  867                 return -EMSGSIZE;
>>>  868         }
>>>
>>> If this solution looks good, I will add this patch in my V2 series.
>> This solves the 1st issue in my report, but without a fix for the 2nd 
>> issue ( vdpa_dev_net_config_fill and vdpa_set_features race) 
>> addressed I don't think it covers all incurred regressions.
>>
>> I've replied the way to reproduce the race. For me it's very obvious 
>> to see in my setup.
> Though I still did not see any errors in my run, but I guess you mean 
> the race condition in set_features(), right?
Yes.
>
> The spec says: The device MUST allow reading of any device-specific 
> configuration field before FEATURES_OK is set by the driver.
>
> So there is no need to check whether driver_features is set in 
> vdpa_get_config_unlocked(). We should remove the code checks
> feature_valid and the code set_features to zero. This conflicts with 
> the spec. And so no race conditions.
I don't disagree with the removal, you can try once more. This proposal 
had been attempted but rejected.

-Siwei



>
> Thanks,
> Zhu Lingshan
>>>
>>> > So what is the race?
>>> You'll see the race if you keep 'vdpa dev config show ...' running 
>>> in a tight loop while launching a VM with the vDPA device under query.
>>>
>> -Siwei
>>
>>>
>>> Thanks
>>> Zhu Lingshan
>>>
>>>>
>>>>
>>>>>          2) process and decoding the device features.
>>>>>
>>>>>
>>>>>              2. There was also another implicit assumption that is 
>>>>> broken by
>>>>>              this patch. There could be a vdpa tool query of 
>>>>> config via
>>>>> vdpa_dev_net_config_fill()->vdpa_get_config_unlocked() that races
>>>>>              with the first vdpa_set_features() call from VMM e.g. 
>>>>> QEMU. Since
>>>>>              the S_FEATURES_OK blocking condition is removed, if 
>>>>> the vdpa tool
>>>>>              query occurs earlier than the first 
>>>>> set_driver_features() call from
>>>>>              VMM, the following code will treat the guest as 
>>>>> legacy and then
>>>>>              trigger an erroneous vdpa_set_features_unlocked(... , 
>>>>> 0) call to
>>>>>              the vdpa driver:
>>>>>
>>>>>               374         /*
>>>>>               375          * Config accesses aren't supposed to 
>>>>> trigger before
>>>>>              features are set.
>>>>>               376          * If it does happen we assume a legacy 
>>>>> guest.
>>>>>               377          */
>>>>>               378         if (!vdev->features_valid)
>>>>>               379 vdpa_set_features_unlocked(vdev, 0);
>>>>>               380         ops->get_config(vdev, offset, buf, len);
>>>>>
>>>>>              Depending on vendor driver's implementation, L380 may 
>>>>> either return
>>>>>              invalid config data (or invalid endianness if on BE) 
>>>>> or only config
>>>>>              fields that are valid in legacy layout. What's more 
>>>>> severe is that,
>>>>>              vdpa tool query in theory shouldn't affect feature 
>>>>> negotiation at
>>>>>              all by making confusing calls to the device, but now 
>>>>> it is possible
>>>>>              with the patch. Fixing this would require more 
>>>>> delicate work on the
>>>>>              other paths involving the cf_lock reader/write 
>>>>> semaphore.
>>>>>
>>>>>              Not sure what you plan to do next, post the fixes for 
>>>>> both issues
>>>>>              and get the community review? Or simply revert the 
>>>>> patch in
>>>>>              question? Let us know.
>>>>>
>>>>>          The spec says:
>>>>>          The device MUST allow reading of any device-specific 
>>>>> configuration
>>>>>          field before FEATURES_OK is set by
>>>>>          the driver. This includes fields which are conditional on 
>>>>> feature bits,
>>>>>          as long as those feature bits are offered
>>>>>          by the device.
>>>>>
>>>>>          so whether FEATURES_OK should not block reading the 
>>>>> device config
>>>>>          space. vdpa_get_config_unlocked() will read the features, 
>>>>> I don't know
>>>>>          why it has a comment:
>>>>>                  /*
>>>>>                   * Config accesses aren't supposed to trigger 
>>>>> before features
>>>>>          are set.
>>>>>                   * If it does happen we assume a legacy guest.
>>>>>                   */
>>>>>
>>>>>          This conflicts with the spec.
>>>>>
>>>>>          vdpa_get_config_unlocked() checks vdev->features_valid, 
>>>>> if not valid,
>>>>>          it will set the drivers_features 0, I think this intends 
>>>>> to prevent
>>>>>          reading random driver_features. This function does not 
>>>>> hold any locks,
>>>>>          and didn't change anything.
>>>>>
>>>>>          So what is the race?
>>>>>          You'll see the race if you keep 'vdpa dev config show 
>>>>> ...' running in a
>>>>>      tight loop while launching a VM with the vDPA device under 
>>>>> query.
>>>>>
>>>>>      -Siwei
>>>>>
>>>>>
>>>>>
>>>>>                  Thanks
>>>>>
>>>>>
>>>>>              Thanks,
>>>>>              -Siwei
>>>>>
>>>>>
>>>>>              On 8/12/2022 3:44 AM, Zhu Lingshan wrote:
>>>>>
>>>>>                  Users may want to query the config space of a 
>>>>> vDPA device,
>>>>>                  to choose a appropriate one for a certain guest. 
>>>>> This means the
>>>>>                  users need to read the config space before 
>>>>> FEATURES_OK, and
>>>>>                  the existence of config space contents does not 
>>>>> depend on
>>>>>                  FEATURES_OK.
>>>>>
>>>>>                  The spec says:
>>>>>                  The device MUST allow reading of any device-specific
>>>>>                  configuration
>>>>>                  field before FEATURES_OK is set by the driver. 
>>>>> This includes
>>>>>                  fields which are conditional on feature bits, as 
>>>>> long as those
>>>>>                  feature bits are offered by the device.
>>>>>
>>>>>                  Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
>>>>>                  ---
>>>>>                    drivers/vdpa/vdpa.c | 8 --------
>>>>>                    1 file changed, 8 deletions(-)
>>>>>
>>>>>                  diff --git a/drivers/vdpa/vdpa.c 
>>>>> b/drivers/vdpa/vdpa.c
>>>>>                  index 6eb3d972d802..bf312d9c59ab 100644
>>>>>                  --- a/drivers/vdpa/vdpa.c
>>>>>                  +++ b/drivers/vdpa/vdpa.c
>>>>>                  @@ -855,17 +855,9 @@ vdpa_dev_config_fill(struct 
>>>>> vdpa_device
>>>>>                  *vdev, struct sk_buff *msg, u32 portid,
>>>>>                    {
>>>>>                        u32 device_id;
>>>>>                        void *hdr;
>>>>>                  -    u8 status;
>>>>>                        int err;
>>>>> down_read(&vdev->cf_lock);
>>>>>                  -    status = vdev->config->get_status(vdev);
>>>>>                  -    if (!(status & VIRTIO_CONFIG_S_FEATURES_OK)) {
>>>>>                  -        NL_SET_ERR_MSG_MOD(extack, "Features 
>>>>> negotiation not
>>>>>                  completed");
>>>>>                  -        err = -EAGAIN;
>>>>>                  -        goto out;
>>>>>                  -    }
>>>>>                  -
>>>>>                        hdr = genlmsg_put(msg, portid, seq, 
>>>>> &vdpa_nl_family,
>>>>>                  flags,
>>>>> VDPA_CMD_DEV_CONFIG_GET);
>>>>>                        if (!hdr) {
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>
>>
>

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