Re: [PATCH net] vhost_net: remove VHOST_NET_F_VIRTIO_NET_HDR support

[Jason Wang <jasowang@redhat.com>]

On 2018年06月11日 10:12, Michael S. Tsirkin wrote:
> On Fri, Jun 08, 2018 at 01:07:09PM +0800, Jason Wang wrote:
>>
>> On 2018年06月08日 12:46, Michael S. Tsirkin wrote:
>>> On Fri, Jun 08, 2018 at 11:50:42AM +0800, Jason Wang wrote:
>>>> This feature bit is duplicated with VIRTIO_F_ANY_LAYOUT, this means if
>>>> a userpsace want to enable VRITIO_F_ANY_LAYOUT,
>>>> VHOST_NET_F_VIRTIO_NET_HDR will be implied too. This is wrong and will
>>>> break networking.
>>> What breaks networking exactly? VHOST_NET supported ANY_LAYOUT
>>> from day one. For this reason it does not need to know about
>>> VRITIO_F_ANY_LAYOUT and we reused the bit for other purposes.
>> It's the knowledge of vhost_net code it self but not userspace. For
>> userspace, it should depends on the value of returned by VHOST_GET_FEATURES.
>> So when userspace can set_features with ANY_LAYOUT, vhost may think it wants
>> VHOST_NET_F_VIRTIO_NET_HDR.
> Yes but that's the admittedly ugly API that we have now.
> userspace is supposed to know VRITIO_F_ANY_LAYOUT does
> not make sense for vhost.

Ok.

>
>
>
>>>
>>>
>>>> Fixing this by safely removing
>>>> VHOST_NET_F_VIRTIO_NET_HDR support. There should be very few or even
>>>> no userspace can use this.
>>> Quite possibly, but it is hard to be sure. It seems safer to
>>> maintain it unless there's an actual reason something's broken.
>> I think not since the feature is negotiated not mandatory?
> That doesn't mean much.
>
>>>> Further cleanups could be done for
>>>> -net-next for safety.
>>>>
>>>> In the future, we need a vhost dedicated feature set/get ioctl()
>>>> instead of reusing virtio ones.
>>> Not just in the future, we might want to switch iommu
>>> to a sane structure without the 64 bit padding bug
>>> right now.
>> Yes, I hit this bug when introducing V2 of msg IOTLB message.
> Sounds good, so if you like, reserve a bit for
> VHOST_NET_F_VIRTIO_NET_HDR in the new ioctl mask and
> do not enable it there.

Ok, and maybe VHOST_F_LOG_ALL.

>
>>>> Fixes: 4e9fa50c6ccbe ("vhost: move features to core")
>>> This tag makes no sense here IMHO. Looks like people are using some tool
>>> that just looks at the earliest version where patch won't apply. The
>>> commit in question just moved some code around.
>> Looks not, before this commit, vhost_net won't return ANY_LAYOUT.
>>
>> Thanks
> Well ANY_LAYOUT just happens to be same as VHOST_NET_F_VIRTIO_NET_HDR
> and that has been set since forever.

So do you still want this patch? If not we need to document that 
ANY_LAYOUT could not be passed through SET_FEATURES somewhere.

Thanks

>
>>>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>>>> ---
>>>>    drivers/vhost/net.c | 15 +++++----------
>>>>    1 file changed, 5 insertions(+), 10 deletions(-)
>>>>
>>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>>>> index 986058a..83eef52 100644
>>>> --- a/drivers/vhost/net.c
>>>> +++ b/drivers/vhost/net.c
>>>> @@ -69,7 +69,6 @@ MODULE_PARM_DESC(experimental_zcopytx, "Enable Zero Copy TX;"
>>>>    enum {
>>>>    	VHOST_NET_FEATURES = VHOST_FEATURES |
>>>> -			 (1ULL << VHOST_NET_F_VIRTIO_NET_HDR) |
>>>>    			 (1ULL << VIRTIO_NET_F_MRG_RXBUF) |
>>>>    			 (1ULL << VIRTIO_F_IOMMU_PLATFORM)
>>>>    };
>>>> @@ -1255,15 +1254,11 @@ static int vhost_net_set_features(struct vhost_net *n, u64 features)
>>>>    			       (1ULL << VIRTIO_F_VERSION_1))) ?
>>>>    			sizeof(struct virtio_net_hdr_mrg_rxbuf) :
>>>>    			sizeof(struct virtio_net_hdr);
>>>> -	if (features & (1 << VHOST_NET_F_VIRTIO_NET_HDR)) {
>>>> -		/* vhost provides vnet_hdr */
>>>> -		vhost_hlen = hdr_len;
>>>> -		sock_hlen = 0;
>>>> -	} else {
>>>> -		/* socket provides vnet_hdr */
>>>> -		vhost_hlen = 0;
>>>> -		sock_hlen = hdr_len;
>>>> -	}
>>>> +
>>>> +        /* socket provides vnet_hdr */
>>>> +	vhost_hlen = 0;
>>>> +	sock_hlen = hdr_len;
>>>> +
>>>>    	mutex_lock(&n->dev.mutex);
>>>>    	if ((features & (1 << VHOST_F_LOG_ALL)) &&
>>>>    	    !vhost_log_access_ok(&n->dev))
>>>> -- 
>>>> 2.7.4