Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature

["Michael S. Tsirkin" <mst@redhat.com>]

Akihiko Odaki sorry about reposts.
Having an email with two "@" in the CC list:
	 xuanzhuo@linux.alibaba.comshuah@kernel.org
tripped up mutt's reply-all for me and made it send to you only.

I am guessing you meant two addresses: xuanzhuo@linux.alibaba.com
and shuah@kernel.org.


On Sun, Oct 08, 2023 at 02:20:49PM +0900, Akihiko Odaki wrote:
> virtio-net have two usage of hashes: one is RSS and another is hash
> reporting. Conventionally the hash calculation was done by the VMM.
> However, computing the hash after the queue was chosen defeats the
> purpose of RSS.
> 
> Another approach is to use eBPF steering program. This approach has
> another downside: it cannot report the calculated hash due to the
> restrictive nature of eBPF.
> 
> Introduce the code to compute hashes to the kernel in order to overcome
> thse challenges. An alternative solution is to extend the eBPF steering
> program so that it will be able to report to the userspace, but it makes
> little sense to allow to implement different hashing algorithms with
> eBPF since the hash value reported by virtio-net is strictly defined by
> the specification.
> 
> The hash value already stored in sk_buff is not used and computed
> independently since it may have been computed in a way not conformant
> with the specification.
> 
> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> ---
>  drivers/net/tun.c           | 187 ++++++++++++++++++++++++++++++++----
>  include/uapi/linux/if_tun.h |  16 +++
>  2 files changed, 182 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 89ab9efe522c..561a573cd008 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -171,6 +171,9 @@ struct tun_prog {
>  	struct bpf_prog *prog;
>  };
>  
> +#define TUN_VNET_HASH_MAX_KEY_SIZE 40
> +#define TUN_VNET_HASH_MAX_INDIRECTION_TABLE_LENGTH 128
> +

where do these come from?

>  /* Since the socket were moved to tun_file, to preserve the behavior of persist
>   * device, socket filter, sndbuf and vnet header size were restore when the
>   * file were attached to a persist device.
> @@ -209,6 +212,9 @@ struct tun_struct {
>  	struct tun_prog __rcu *steering_prog;
>  	struct tun_prog __rcu *filter_prog;
>  	struct ethtool_link_ksettings link_ksettings;
> +	struct tun_vnet_hash vnet_hash;
> +	u16 vnet_hash_indirection_table[TUN_VNET_HASH_MAX_INDIRECTION_TABLE_LENGTH];
> +	u32 vnet_hash_key[TUN_VNET_HASH_MAX_KEY_SIZE / 4];

That's quite a lot of data to add in this struct, and will be used
by a small minority of users. Are you pushing any hot data out of cache
with this? Why not allocate these as needed?

>  	/* init args */
>  	struct file *file;
>  	struct ifreq *ifr;
> @@ -219,6 +225,13 @@ struct veth {
>  	__be16 h_vlan_TCI;
>  };
>  
> +static const struct tun_vnet_hash_cap tun_vnet_hash_cap = {
> +	.max_indirection_table_length =
> +		TUN_VNET_HASH_MAX_INDIRECTION_TABLE_LENGTH,
> +
> +	.types = VIRTIO_NET_SUPPORTED_HASH_TYPES
> +};
> +
>  static void tun_flow_init(struct tun_struct *tun);
>  static void tun_flow_uninit(struct tun_struct *tun);
>  
> @@ -320,10 +333,16 @@ static long tun_set_vnet_be(struct tun_struct *tun, int __user *argp)
>  	if (get_user(be, argp))
>  		return -EFAULT;
>  
> -	if (be)
> +	if (be) {
> +		if (!(tun->flags & TUN_VNET_LE) &&
> +		    (tun->vnet_hash.flags & TUN_VNET_HASH_REPORT)) {
> +			return -EINVAL;
> +		}
> +
>  		tun->flags |= TUN_VNET_BE;
> -	else
> +	} else {
>  		tun->flags &= ~TUN_VNET_BE;
> +	}
>  
>  	return 0;
>  }
> @@ -558,15 +577,47 @@ static u16 tun_ebpf_select_queue(struct tun_struct *tun, struct sk_buff *skb)
>  	return ret % numqueues;
>  }
>  
> +static u16 tun_vnet_select_queue(struct tun_struct *tun, struct sk_buff *skb)
> +{
> +	u32 value = qdisc_skb_cb(skb)->tun_vnet_hash_value;
> +	u32 numqueues;
> +	u32 index;
> +	u16 queue;
> +
> +	numqueues = READ_ONCE(tun->numqueues);
> +	if (!numqueues)
> +		return 0;
> +
> +	index = value & READ_ONCE(tun->vnet_hash.indirection_table_mask);
> +	queue = READ_ONCE(tun->vnet_hash_indirection_table[index]);
> +	if (!queue)
> +		queue = READ_ONCE(tun->vnet_hash.unclassified_queue);

Apparently 0 is an illegal queue value? You are making this part
of UAPI better document things like this.

> +
> +	return queue % numqueues;
> +}
> +
>  static u16 tun_select_queue(struct net_device *dev, struct sk_buff *skb,
>  			    struct net_device *sb_dev)
>  {
>  	struct tun_struct *tun = netdev_priv(dev);
> +	u8 vnet_hash_flags = READ_ONCE(tun->vnet_hash.flags);
> +	struct virtio_net_hash hash;
>  	u16 ret;
>  
> +	if (vnet_hash_flags & (TUN_VNET_HASH_RSS | TUN_VNET_HASH_REPORT)) {
> +		virtio_net_hash(skb, READ_ONCE(tun->vnet_hash.types),
> +				tun->vnet_hash_key, &hash);
> +

What are all these READ_ONCE things doing?
E.g. you seem to be very content to have tun->vnet_hash.types read twice apparently?
This is volatile which basically breaks all compiler's attempts
to optimize code - needs to be used judiciously.



> +		skb->tun_vnet_hash = true;
> +		qdisc_skb_cb(skb)->tun_vnet_hash_value = hash.value;
> +		qdisc_skb_cb(skb)->tun_vnet_hash_report = hash.report;
> +	}
> +
>  	rcu_read_lock();
>  	if (rcu_dereference(tun->steering_prog))
>  		ret = tun_ebpf_select_queue(tun, skb);
> +	else if (vnet_hash_flags & TUN_VNET_HASH_RSS)
> +		ret = tun_vnet_select_queue(tun, skb);
>  	else
>  		ret = tun_automq_select_queue(tun, skb);
>  	rcu_read_unlock();
> @@ -2088,10 +2139,15 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>  			    struct iov_iter *iter)
>  {
>  	struct tun_pi pi = { 0, skb->protocol };
> +	struct virtio_net_hash vnet_hash = {
> +		.value = qdisc_skb_cb(skb)->tun_vnet_hash_value,
> +		.report = qdisc_skb_cb(skb)->tun_vnet_hash_report
> +	};
>  	ssize_t total;
>  	int vlan_offset = 0;
>  	int vlan_hlen = 0;
>  	int vnet_hdr_sz = 0;
> +	size_t vnet_hdr_content_sz;
>  
>  	if (skb_vlan_tag_present(skb))
>  		vlan_hlen = VLAN_HLEN;
> @@ -2116,31 +2172,49 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>  	}
>  
>  	if (vnet_hdr_sz) {
> -		struct virtio_net_hdr gso;
> +		union {
> +			struct virtio_net_hdr hdr;
> +			struct virtio_net_hdr_v1_hash v1_hash_hdr;
> +		} hdr;
> +		int ret;
>  
>  		if (iov_iter_count(iter) < vnet_hdr_sz)
>  			return -EINVAL;
>  
> -		if (virtio_net_hdr_from_skb(skb, &gso,
> -					    tun_is_little_endian(tun), true,
> -					    vlan_hlen)) {
> +		if ((READ_ONCE(tun->vnet_hash.flags) & TUN_VNET_HASH_REPORT) &&
> +		    vnet_hdr_sz >= sizeof(hdr.v1_hash_hdr) &&
> +		    skb->tun_vnet_hash) {
> +			vnet_hdr_content_sz = sizeof(hdr.v1_hash_hdr);
> +			ret = virtio_net_hdr_v1_hash_from_skb(skb,
> +							      &hdr.v1_hash_hdr,
> +							      true,
> +							      vlan_hlen,
> +							      &vnet_hash);
> +		} else {
> +			vnet_hdr_content_sz = sizeof(hdr.hdr);
> +			ret = virtio_net_hdr_from_skb(skb, &hdr.hdr,
> +						      tun_is_little_endian(tun),
> +						      true, vlan_hlen);
> +		}
> +
> +		if (ret) {
>  			struct skb_shared_info *sinfo = skb_shinfo(skb);
>  			pr_err("unexpected GSO type: "
>  			       "0x%x, gso_size %d, hdr_len %d\n",
> -			       sinfo->gso_type, tun16_to_cpu(tun, gso.gso_size),
> -			       tun16_to_cpu(tun, gso.hdr_len));
> +			       sinfo->gso_type, tun16_to_cpu(tun, hdr.hdr.gso_size),
> +			       tun16_to_cpu(tun, hdr.hdr.hdr_len));
>  			print_hex_dump(KERN_ERR, "tun: ",
>  				       DUMP_PREFIX_NONE,
>  				       16, 1, skb->head,
> -				       min((int)tun16_to_cpu(tun, gso.hdr_len), 64), true);
> +				       min((int)tun16_to_cpu(tun, hdr.hdr.hdr_len), 64), true);
>  			WARN_ON_ONCE(1);
>  			return -EINVAL;
>  		}
>  
> -		if (copy_to_iter(&gso, sizeof(gso), iter) != sizeof(gso))
> +		if (copy_to_iter(&hdr, vnet_hdr_content_sz, iter) != vnet_hdr_content_sz)
>  			return -EFAULT;
>  
> -		iov_iter_advance(iter, vnet_hdr_sz - sizeof(gso));
> +		iov_iter_advance(iter, vnet_hdr_sz - vnet_hdr_content_sz);
>  	}
>  
>  	if (vlan_hlen) {
> @@ -3007,24 +3081,27 @@ static int tun_set_queue(struct file *file, struct ifreq *ifr)
>  	return ret;
>  }
>  
> -static int tun_set_ebpf(struct tun_struct *tun, struct tun_prog __rcu **prog_p,
> -			void __user *data)
> +static struct bpf_prog *tun_set_ebpf(struct tun_struct *tun,
> +				     struct tun_prog __rcu **prog_p,
> +				     void __user *data)
>  {
>  	struct bpf_prog *prog;
>  	int fd;
> +	int ret;
>  
>  	if (copy_from_user(&fd, data, sizeof(fd)))
> -		return -EFAULT;
> +		return ERR_PTR(-EFAULT);
>  
>  	if (fd == -1) {
>  		prog = NULL;
>  	} else {
>  		prog = bpf_prog_get_type(fd, BPF_PROG_TYPE_SOCKET_FILTER);
>  		if (IS_ERR(prog))
> -			return PTR_ERR(prog);
> +			return prog;
>  	}
>  
> -	return __tun_set_ebpf(tun, prog_p, prog);
> +	ret = __tun_set_ebpf(tun, prog_p, prog);
> +	return ret ? ERR_PTR(ret) : prog;
>  }
>  
>  /* Return correct value for tun->dev->addr_len based on tun->dev->type. */
> @@ -3082,6 +3159,11 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
>  	int le;
>  	int ret;
>  	bool do_notify = false;
> +	struct bpf_prog *bpf_ret;
> +	struct tun_vnet_hash vnet_hash;
> +	u16 vnet_hash_indirection_table[TUN_VNET_HASH_MAX_INDIRECTION_TABLE_LENGTH];
> +	u8 vnet_hash_key[TUN_VNET_HASH_MAX_KEY_SIZE];
> +	size_t len;
>  
>  	if (cmd == TUNSETIFF || cmd == TUNSETQUEUE ||
>  	    (_IOC_TYPE(cmd) == SOCK_IOC_TYPE && cmd != SIOCGSKNS)) {
> @@ -3295,7 +3377,10 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
>  			ret = -EFAULT;
>  			break;
>  		}
> -		if (vnet_hdr_sz < (int)sizeof(struct virtio_net_hdr)) {
> +		if (vnet_hdr_sz <
> +		    (int)((tun->vnet_hash.flags & TUN_VNET_HASH_REPORT) ?
> +			  sizeof(struct virtio_net_hdr_v1_hash) :
> +			  sizeof(struct virtio_net_hdr))) {
>  			ret = -EINVAL;
>  			break;
>  		}
> @@ -3314,10 +3399,16 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
>  			ret = -EFAULT;
>  			break;
>  		}
> -		if (le)
> +		if (le) {
>  			tun->flags |= TUN_VNET_LE;
> -		else
> +		} else {
> +			if (!tun_legacy_is_little_endian(tun)) {
> +				ret = -EINVAL;
> +				break;
> +			}
> +
>  			tun->flags &= ~TUN_VNET_LE;
> +		}
>  		break;
>  
>  	case TUNGETVNETBE:
> @@ -3360,11 +3451,17 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
>  		break;
>  
>  	case TUNSETSTEERINGEBPF:
> -		ret = tun_set_ebpf(tun, &tun->steering_prog, argp);
> +		bpf_ret = tun_set_ebpf(tun, &tun->steering_prog, argp);
> +		if (IS_ERR(bpf_ret))
> +			ret = PTR_ERR(bpf_ret);
> +		else if (bpf_ret)
> +			tun->vnet_hash.flags &= ~TUN_VNET_HASH_RSS;

what is this doing?

>  		break;
>  
>  	case TUNSETFILTEREBPF:
> -		ret = tun_set_ebpf(tun, &tun->filter_prog, argp);
> +		bpf_ret = tun_set_ebpf(tun, &tun->filter_prog, argp);
> +		if (IS_ERR(bpf_ret))
> +			ret = PTR_ERR(bpf_ret);
>  		break;
>  
>  	case TUNSETCARRIER:
> @@ -3382,6 +3479,54 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
>  		ret = open_related_ns(&net->ns, get_net_ns);
>  		break;
>  
> +	case TUNGETVNETHASHCAP:
> +		if (copy_to_user(argp, &tun_vnet_hash_cap,
> +				 sizeof(tun_vnet_hash_cap)))
> +			ret = -EFAULT;
> +		break;
> +
> +	case TUNSETVNETHASH:
> +		len = sizeof(vnet_hash);
> +		if (copy_from_user(&vnet_hash, argp, len)) {
> +			ret = -EFAULT;
> +			break;
> +		}


what if flags has some bits set you don't know how to handle?
should these be ignored as now or cause a failure?
these decisions all affect uapi.

> +
> +		if (((vnet_hash.flags & TUN_VNET_HASH_REPORT) &&
> +		     (tun->vnet_hdr_sz < sizeof(struct virtio_net_hdr_v1_hash) ||
> +		      !tun_is_little_endian(tun))) ||
> +		     vnet_hash.indirection_table_mask >=
> +		     TUN_VNET_HASH_MAX_INDIRECTION_TABLE_LENGTH) {
> +			ret = -EINVAL;
> +			break;
> +		}

Given this is later used to index within an array one has to
be very careful about spectre things here, which this code isn't.


> +
> +		argp = (u8 __user *)argp + len;
> +		len = (vnet_hash.indirection_table_mask + 1) * 2;

comment pointer math tricks like this extensively please.

> +		if (copy_from_user(vnet_hash_indirection_table, argp, len)) {
> +			ret = -EFAULT;
> +			break;
> +		}
> +
> +		argp = (u8 __user *)argp + len;
> +		len = virtio_net_hash_key_length(vnet_hash.types);
> +
> +		if (copy_from_user(vnet_hash_key, argp, len)) {
> +			ret = -EFAULT;
> +			break;
> +		}
> +
> +		tun->vnet_hash = vnet_hash;
> +		memcpy(tun->vnet_hash_indirection_table,
> +		       vnet_hash_indirection_table,
> +		       (vnet_hash.indirection_table_mask + 1) * 2);
> +		memcpy(tun->vnet_hash_key, vnet_hash_key, len);
> +
> +		if (vnet_hash.flags & TUN_VNET_HASH_RSS)
> +			__tun_set_ebpf(tun, &tun->steering_prog, NULL);
> +
> +		break;
> +
>  	default:
>  		ret = -EINVAL;
>  		break;
> diff --git a/include/uapi/linux/if_tun.h b/include/uapi/linux/if_tun.h
> index 287cdc81c939..dc591cd897c8 100644
> --- a/include/uapi/linux/if_tun.h
> +++ b/include/uapi/linux/if_tun.h
> @@ -61,6 +61,8 @@
>  #define TUNSETFILTEREBPF _IOR('T', 225, int)
>  #define TUNSETCARRIER _IOW('T', 226, int)
>  #define TUNGETDEVNETNS _IO('T', 227)
> +#define TUNGETVNETHASHCAP _IO('T', 228)
> +#define TUNSETVNETHASH _IOW('T', 229, unsigned int)
>  
>  /* TUNSETIFF ifr flags */
>  #define IFF_TUN		0x0001
> @@ -115,4 +117,18 @@ struct tun_filter {
>  	__u8   addr[][ETH_ALEN];
>  };
>  
> +struct tun_vnet_hash_cap {
> +	__u16 max_indirection_table_length;
> +	__u32 types;
> +};
> +

There's hidden padding in this struct - not good, copy
will leak kernel info out.



> +#define TUN_VNET_HASH_RSS	0x01
> +#define TUN_VNET_HASH_REPORT	0x02


Do you intend to add more flags down the road?
How will userspace know what is supported?

> +struct tun_vnet_hash {
> +	__u8 flags;
> +	__u32 types;
> +	__u16 indirection_table_mask;
> +	__u16 unclassified_queue;
> +};
> +

Padding here too. Best avoided.

In any case, document UAPI please.


>  #endif /* _UAPI__IF_TUN_H */
> -- 
> 2.42.0