bpf: document answers to common questions about BPF

to address common misconceptions about what BPF is and what it's not
add short BPF Q&A that clarifies core BPF design principles and
answers some common questions.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/bpf/bpf_design_QA.txt

@@ -0,0 +1,156 @@
+BPF extensibility and applicability to networking, tracing, security
+in the linux kernel and several user space implementations of BPF
+virtual machine led to a number of misunderstanding on what BPF actually is.
+This short QA is an attempt to address that and outline a direction
+of where BPF is heading long term.
+
+Q: Is BPF a generic instruction set similar to x64 and arm64?
+A: NO.
+
+Q: Is BPF a generic virtual machine ?
+A: NO.
+
+BPF is generic instruction set _with_ C calling convention.
+
+Q: Why C calling convention was chosen?
+A: Because BPF programs are designed to run in the linux kernel
+   which is written in C, hence BPF defines instruction set compatible
+   with two most used architectures x64 and arm64 (and takes into
+   consideration important quirks of other architectures) and
+   defines calling convention that is compatible with C calling
+   convention of the linux kernel on those architectures.
+
+Q: can multiple return values be supported in the future?
+A: NO. BPF allows only register R0 to be used as return value.
+
+Q: can more than 5 function arguments be supported in the future?
+A: NO. BPF calling convention only allows registers R1-R5 to be used
+   as arguments. BPF is not a standalone instruction set.
+   (unlike x64 ISA that allows msft, cdecl and other conventions)
+
+Q: can BPF programs access instruction pointer or return address?
+A: NO.
+
+Q: can BPF programs access stack pointer ?
+A: NO. Only frame pointer (register R10) is accessible.
+   From compiler point of view it's necessary to have stack pointer.
+   For example LLVM defines register R11 as stack pointer in its
+   BPF backend, but it makes sure that generated code never uses it.
+
+Q: Does C-calling convention diminishes possible use cases?
+A: YES. BPF design forces addition of major functionality in the form
+   of kernel helper functions and kernel objects like BPF maps with
+   seamless interoperability between them. It lets kernel call into
+   BPF programs and programs call kernel helpers with zero overhead.
+   As all of them were native C code. That is particularly the case
+   for JITed BPF programs that are indistinguishable from
+   native kernel C code.
+
+Q: Does it mean that 'innovative' extensions to BPF code are disallowed?
+A: Soft yes. At least for now until BPF core has support for
+   bpf-to-bpf calls, indirect calls, loops, global variables,
+   jump tables, read only sections and all other normal constructs
+   that C code can produce.
+
+Q: Can loops be supported in a safe way?
+A: It's not clear yet. BPF developers are trying to find a way to
+   support bounded loops where the verifier can guarantee that
+   the program terminates in less than 4096 instructions.
+
+Q: How come LD_ABS and LD_IND instruction are present in BPF whereas
+   C code cannot express them and has to use builtin intrinsics?
+A: This is artifact of compatibility with classic BPF. Modern
+   networking code in BPF performs better without them.
+   See 'direct packet access'.
+
+Q: It seems not all BPF instructions are one-to-one to native CPU.
+   For example why BPF_JNE and other compare and jumps are not cpu-like?
+A: This was necessary to avoid introducing flags into ISA which are
+   impossible to make generic and efficient across CPU architectures.
+
+Q: why BPF_DIV instruction doesn't map to x64 div?
+A: Because if we picked one-to-one relationship to x64 it would have made
+   it more complicated to support on arm64 and other archs. Also it
+   needs div-by-zero runtime check.
+
+Q: why there is no BPF_SDIV for signed divide operation?
+A: Because it would be rarely used. llvm errors in such case and
+   prints a suggestion to use unsigned divide instead
+
+Q: Why BPF has implicit prologue and epilogue?
+A: Because architectures like sparc have register windows and in general
+   there are enough subtle differences between architectures, so naive
+   store return address into stack won't work. Another reason is BPF has
+   to be safe from division by zero (and legacy exception path
+   of LD_ABS insn). Those instructions need to invoke epilogue and
+   return implicitly.
+
+Q: Why BPF_JLT and BPF_JLE instructions were not introduced in the beginning?
+A: Because classic BPF didn't have them and BPF authors felt that compiler
+   workaround would be acceptable. Turned out that programs lose performance
+   due to lack of these compare instructions and they were added.
+   These two instructions is a perfect example what kind of new BPF
+   instructions are acceptable and can be added in the future.
+   These two already had equivalent instructions in native CPUs.
+   New instructions that don't have one-to-one mapping to HW instructions
+   will not be accepted.
+
+Q: BPF 32-bit subregisters have a requirement to zero upper 32-bits of BPF
+   registers which makes BPF inefficient virtual machine for 32-bit
+   CPU architectures and 32-bit HW accelerators. Can true 32-bit registers
+   be added to BPF in the future?
+A: NO. The first thing to improve performance on 32-bit archs is to teach
+   LLVM to generate code that uses 32-bit subregisters. Then second step
+   is to teach verifier to mark operations where zero-ing upper bits
+   is unnecessary. Then JITs can take advantage of those markings and
+   drastically reduce size of generated code and improve performance.
+
+Q: Does BPF have a stable ABI?
+A: YES. BPF instructions, arguments to BPF programs, set of helper
+   functions and their arguments, recognized return codes are all part
+   of ABI. However when tracing programs are using bpf_probe_read() helper
+   to walk kernel internal datastructures and compile with kernel
+   internal headers these accesses can and will break with newer
+   kernels. The union bpf_attr -> kern_version is checked at load time
+   to prevent accidentally loading kprobe-based bpf programs written
+   for a different kernel. Networking programs don't do kern_version check.
+
+Q: How much stack space a BPF program uses?
+A: Currently all program types are limited to 512 bytes of stack
+   space, but the verifier computes the actual amount of stack used
+   and both interpreter and most JITed code consume necessary amount.
+
+Q: Can BPF be offloaded to HW?
+A: YES. BPF HW offload is supported by NFP driver.
+
+Q: Does classic BPF interpreter still exist?
+A: NO. Classic BPF programs are converted into extend BPF instructions.
+
+Q: Can BPF call arbitrary kernel functions?
+A: NO. BPF programs can only call a set of helper functions which
+   is defined for every program type.
+
+Q: Can BPF overwrite arbitrary kernel memory?
+A: NO. Tracing bpf programs can _read_ arbitrary memory with bpf_probe_read()
+   and bpf_probe_read_str() helpers. Networking programs cannot read
+   arbitrary memory, since they don't have access to these helpers.
+   Programs can never read or write arbitrary memory directly.
+
+Q: Can BPF overwrite arbitrary user memory?
+A: Sort-of. Tracing BPF programs can overwrite the user memory
+   of the current task with bpf_probe_write_user(). Every time such
+   program is loaded the kernel will print warning message, so
+   this helper is only useful for experiments and prototypes.
+   Tracing BPF programs are root only.
+
+Q: When bpf_trace_printk() helper is used the kernel prints nasty
+   warning message. Why is that?
+A: This is done to nudge program authors into better interfaces when
+   programs need to pass data to user space. Like bpf_perf_event_output()
+   can be used to efficiently stream data via perf ring buffer.
+   BPF maps can be used for asynchronous data sharing between kernel
+   and user space. bpf_trace_printk() should only be used for debugging.
+
+Q: Can BPF functionality such as new program or map types, new
+   helpers, etc be added out of kernel module code?
+A: NO.

MAINTAINERS

@@ -2713,6 +2713,7 @@ L:	linux-kernel@vger.kernel.org
 S:	Supported
 F:	arch/x86/net/bpf_jit*
 F:	Documentation/networking/filter.txt
+F:	Documentation/bpf/
 F:	include/linux/bpf*
 F:	include/linux/filter.h
 F:	include/uapi/linux/bpf*