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diff --git a/Documentation/bpf/bpf_design_QA.txt b/Documentation/bpf/bpf_design_QA.txt deleted file mode 100644 index f3e458a0bb2f1..0000000000000 --- a/Documentation/bpf/bpf_design_QA.txt +++ /dev/null @@ -1,156 +0,0 @@ -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. |