From 46604676c8c6c4c07649767d32ae66f4429ccd6f Mon Sep 17 00:00:00 2001 From: Andrii Nakryiko Date: Thu, 28 Feb 2019 17:12:21 -0800 Subject: [PATCH] docs/bpf: minor casing/punctuation fixes Fix few casing and punctuation glitches. Signed-off-by: Andrii Nakryiko Signed-off-by: Daniel Borkmann --- Documentation/bpf/bpf_design_QA.rst | 24 ++++++++++++------------ Documentation/networking/filter.txt | 2 +- 2 files changed, 13 insertions(+), 13 deletions(-) diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst index 7cc9e368c1e9..10453c627135 100644 --- a/Documentation/bpf/bpf_design_QA.rst +++ b/Documentation/bpf/bpf_design_QA.rst @@ -36,27 +36,27 @@ 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? +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? +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? +Q: Can BPF programs access instruction pointer or return address? ----------------------------------------------------------------- A: NO. -Q: can BPF programs access stack pointer ? +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 +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? @@ -66,8 +66,8 @@ 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 +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. @@ -75,9 +75,9 @@ 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 +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 +jump tables, read-only sections, and all other normal constructs that C code can produce. Q: Can loops be supported in a safe way? @@ -109,16 +109,16 @@ 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? +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? +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 +prints a suggestion to use unsigned divide instead. Q: Why BPF has implicit prologue and epilogue? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt index b5e060edfc38..319e5e041f38 100644 --- a/Documentation/networking/filter.txt +++ b/Documentation/networking/filter.txt @@ -829,7 +829,7 @@ tracing filters may do to maintain counters of events, for example. Register R9 is not used by socket filters either, but more complex filters may be running out of registers and would have to resort to spill/fill to stack. -Internal BPF can used as generic assembler for last step performance +Internal BPF can be used as a generic assembler for last step performance optimizations, socket filters and seccomp are using it as assembler. Tracing filters may use it as assembler to generate code from kernel. In kernel usage may not be bounded by security considerations, since generated internal BPF code