docs/bpf: minor casing/punctuation fixes
Fix few casing and punctuation glitches. Signed-off-by: Andrii Nakryiko <andriin@fb.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
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@ -36,27 +36,27 @@ consideration important quirks of other architectures) and
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defines calling convention that is compatible with C calling
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convention of the linux kernel on those architectures.
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Q: can multiple return values be supported in the future?
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Q: Can multiple return values be supported in the future?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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A: NO. BPF allows only register R0 to be used as return value.
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Q: can more than 5 function arguments be supported in the future?
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Q: Can more than 5 function arguments be supported in the future?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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A: NO. BPF calling convention only allows registers R1-R5 to be used
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as arguments. BPF is not a standalone instruction set.
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(unlike x64 ISA that allows msft, cdecl and other conventions)
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Q: can BPF programs access instruction pointer or return address?
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Q: Can BPF programs access instruction pointer or return address?
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-----------------------------------------------------------------
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A: NO.
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Q: can BPF programs access stack pointer ?
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Q: Can BPF programs access stack pointer ?
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------------------------------------------
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A: NO.
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Only frame pointer (register R10) is accessible.
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From compiler point of view it's necessary to have stack pointer.
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For example LLVM defines register R11 as stack pointer in its
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For example, LLVM defines register R11 as stack pointer in its
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BPF backend, but it makes sure that generated code never uses it.
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Q: Does C-calling convention diminishes possible use cases?
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@ -66,8 +66,8 @@ A: YES.
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BPF design forces addition of major functionality in the form
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of kernel helper functions and kernel objects like BPF maps with
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seamless interoperability between them. It lets kernel call into
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BPF programs and programs call kernel helpers with zero overhead.
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As all of them were native C code. That is particularly the case
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BPF programs and programs call kernel helpers with zero overhead,
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as all of them were native C code. That is particularly the case
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for JITed BPF programs that are indistinguishable from
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native kernel C code.
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@ -75,9 +75,9 @@ Q: Does it mean that 'innovative' extensions to BPF code are disallowed?
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------------------------------------------------------------------------
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A: Soft yes.
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At least for now until BPF core has support for
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At least for now, until BPF core has support for
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bpf-to-bpf calls, indirect calls, loops, global variables,
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jump tables, read only sections and all other normal constructs
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jump tables, read-only sections, and all other normal constructs
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that C code can produce.
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Q: Can loops be supported in a safe way?
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@ -109,16 +109,16 @@ For example why BPF_JNE and other compare and jumps are not cpu-like?
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A: This was necessary to avoid introducing flags into ISA which are
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impossible to make generic and efficient across CPU architectures.
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Q: why BPF_DIV instruction doesn't map to x64 div?
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Q: Why BPF_DIV instruction doesn't map to x64 div?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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A: Because if we picked one-to-one relationship to x64 it would have made
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it more complicated to support on arm64 and other archs. Also it
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needs div-by-zero runtime check.
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Q: why there is no BPF_SDIV for signed divide operation?
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Q: Why there is no BPF_SDIV for signed divide operation?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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A: Because it would be rarely used. llvm errors in such case and
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prints a suggestion to use unsigned divide instead
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prints a suggestion to use unsigned divide instead.
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Q: Why BPF has implicit prologue and epilogue?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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@ -829,7 +829,7 @@ tracing filters may do to maintain counters of events, for example. Register R9
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is not used by socket filters either, but more complex filters may be running
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out of registers and would have to resort to spill/fill to stack.
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Internal BPF can used as generic assembler for last step performance
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Internal BPF can be used as a generic assembler for last step performance
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optimizations, socket filters and seccomp are using it as assembler. Tracing
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filters may use it as assembler to generate code from kernel. In kernel usage
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may not be bounded by security considerations, since generated internal BPF code
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