overall average bitrate for the stream and keeps it below or equal to the set bitrate. In the first case
the average bitrate for the whole stream will be smaller then the set bitrate. This is caused because the
average is calculated for smaller number of frames, on the other hand enabling this setting will ensure that
- the stream will meet tight bandwidth contraints. Applicable to encoders.
+ the stream will meet tight bandwidth constraints. Applicable to encoders.
</entry>
</row>
<row><entry></entry></row>
<section id="flash-controls">
<title>Flash Control Reference</title>
- <note>
- <title>Experimental</title>
-
- <para>This is an <link linkend="experimental">experimental</link>
-interface and may change in the future.</para>
- </note>
-
<para>
The V4L2 flash controls are intended to provide generic access
to flash controller devices. Flash controller devices are
<section id="image-source-controls">
<title>Image Source Control Reference</title>
- <note>
- <title>Experimental</title>
-
- <para>This is an <link
- linkend="experimental">experimental</link> interface and may
- change in the future.</para>
- </note>
-
<para>
The Image Source control class is intended for low-level
control of image source devices such as image sensors. The
<section id="image-process-controls">
<title>Image Process Control Reference</title>
- <note>
- <title>Experimental</title>
-
- <para>This is an <link
- linkend="experimental">experimental</link> interface and may
- change in the future.</para>
- </note>
-
<para>
The Image Process control class is intended for low-level control of
image processing functions. Unlike
<section id="dv-controls">
<title>Digital Video Control Reference</title>
- <note>
- <title>Experimental</title>
-
- <para>This is an <link
- linkend="experimental">experimental</link> interface and may
- change in the future.</para>
- </note>
-
<para>
The Digital Video control class is intended to control receivers
and transmitters for <ulink url="http://en.wikipedia.org/wiki/Vga">VGA</ulink>,
jmp 6 Jump to label
ja 6 Jump to label
- jeq 7, 8 Jump on k == A
- jneq 8 Jump on k != A
- jne 8 Jump on k != A
- jlt 8 Jump on k < A
- jle 8 Jump on k <= A
- jgt 7, 8 Jump on k > A
- jge 7, 8 Jump on k >= A
- jset 7, 8 Jump on k & A
+ jeq 7, 8 Jump on A == k
+ jneq 8 Jump on A != k
+ jne 8 Jump on A != k
+ jlt 8 Jump on A < k
+ jle 8 Jump on A <= k
+ jgt 7, 8 Jump on A > k
+ jge 7, 8 Jump on A >= k
+ jset 7, 8 Jump on A & k
add 0, 4 A + <x>
sub 0, 4 A - <x>
mul 0, 4 A * <x>
div 0, 4 A / <x>
mod 0, 4 A % <x>
- neg 0, 4 !A
+ neg !A
and 0, 4 A & <x>
or 0, 4 A | <x>
xor 0, 4 A ^ <x>
See details of eBPF verifier in kernel/bpf/verifier.c
+Direct packet access
+--------------------
+In cls_bpf and act_bpf programs the verifier allows direct access to the packet
+data via skb->data and skb->data_end pointers.
+Ex:
+1: r4 = *(u32 *)(r1 +80) /* load skb->data_end */
+2: r3 = *(u32 *)(r1 +76) /* load skb->data */
+3: r5 = r3
+4: r5 += 14
+5: if r5 > r4 goto pc+16
+R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp
+6: r0 = *(u16 *)(r3 +12) /* access 12 and 13 bytes of the packet */
+
+this 2byte load from the packet is safe to do, since the program author
+did check 'if (skb->data + 14 > skb->data_end) goto err' at insn #5 which
+means that in the fall-through case the register R3 (which points to skb->data)
+has at least 14 directly accessible bytes. The verifier marks it
+as R3=pkt(id=0,off=0,r=14).
+id=0 means that no additional variables were added to the register.
+off=0 means that no additional constants were added.
+r=14 is the range of safe access which means that bytes [R3, R3 + 14) are ok.
+Note that R5 is marked as R5=pkt(id=0,off=14,r=14). It also points
+to the packet data, but constant 14 was added to the register, so
+it now points to 'skb->data + 14' and accessible range is [R5, R5 + 14 - 14)
+which is zero bytes.
+
+More complex packet access may look like:
+ R0=imm1 R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp
+ 6: r0 = *(u8 *)(r3 +7) /* load 7th byte from the packet */
+ 7: r4 = *(u8 *)(r3 +12)
+ 8: r4 *= 14
+ 9: r3 = *(u32 *)(r1 +76) /* load skb->data */
+10: r3 += r4
+11: r2 = r1
+12: r2 <<= 48
+13: r2 >>= 48
+14: r3 += r2
+15: r2 = r3
+16: r2 += 8
+17: r1 = *(u32 *)(r1 +80) /* load skb->data_end */
+18: if r2 > r1 goto pc+2
+ R0=inv56 R1=pkt_end R2=pkt(id=2,off=8,r=8) R3=pkt(id=2,off=0,r=8) R4=inv52 R5=pkt(id=0,off=14,r=14) R10=fp
+19: r1 = *(u8 *)(r3 +4)
+The state of the register R3 is R3=pkt(id=2,off=0,r=8)
+id=2 means that two 'r3 += rX' instructions were seen, so r3 points to some
+offset within a packet and since the program author did
+'if (r3 + 8 > r1) goto err' at insn #18, the safe range is [R3, R3 + 8).
+The verifier only allows 'add' operation on packet registers. Any other
+operation will set the register state to 'unknown_value' and it won't be
+available for direct packet access.
+Operation 'r3 += rX' may overflow and become less than original skb->data,
+therefore the verifier has to prevent that. So it tracks the number of
+upper zero bits in all 'uknown_value' registers, so when it sees
+'r3 += rX' instruction and rX is more than 16-bit value, it will error as:
+"cannot add integer value with N upper zero bits to ptr_to_packet"
+Ex. after insn 'r4 = *(u8 *)(r3 +12)' (insn #7 above) the state of r4 is
+R4=inv56 which means that upper 56 bits on the register are guaranteed
+to be zero. After insn 'r4 *= 14' the state becomes R4=inv52, since
+multiplying 8-bit value by constant 14 will keep upper 52 bits as zero.
+Similarly 'r2 >>= 48' will make R2=inv48, since the shift is not sign
+extending. This logic is implemented in evaluate_reg_alu() function.
+
+The end result is that bpf program author can access packet directly
+using normal C code as:
+ void *data = (void *)(long)skb->data;
+ void *data_end = (void *)(long)skb->data_end;
+ struct eth_hdr *eth = data;
+ struct iphdr *iph = data + sizeof(*eth);
+ struct udphdr *udp = data + sizeof(*eth) + sizeof(*iph);
+
+ if (data + sizeof(*eth) + sizeof(*iph) + sizeof(*udp) > data_end)
+ return 0;
+ if (eth->h_proto != htons(ETH_P_IP))
+ return 0;
+ if (iph->protocol != IPPROTO_UDP || iph->ihl != 5)
+ return 0;
+ if (udp->dest == 53 || udp->source == 9)
+ ...;
+which makes such programs easier to write comparing to LD_ABS insn
+and significantly faster.
+
eBPF maps
---------
'maps' is a generic storage of different types for sharing data between kernel
the underlying architecture.
-Alexei Starovoitov <ast@plumgrid.com>
+Alexei Starovoitov <ast@kernel.org>
projects / linux.git / commitdiff