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.Dd February 27, 2023
.Dt DTRACE_KINST 4
.Os
.Sh NAME
.Nm dtrace_kinst
.Nd a DTrace provider for tracing arbitrary instructions in a given kernel function
.Sh SYNOPSIS
kinst::<function>:<instruction>
.Sh DESCRIPTION
The DTrace
.Nm kinst
provider allows the user to trace any instruction in a given kernel function.
<function> corresponds to the function to be traced, and <instruction> is the
offset to the specific instruction, and can be obtained from the function's
disassembly using kgdb from the gdb package.
.Pp
.Nm kinst
creates probes on-demand, meaning it searches for and parses the function's
instructions each time
.Xr dtrace 1
is run, and not at module load time.
This is in contrast to FBT's load-time parsing, since
.Nm kinst
can potentially create thousands of probes for just a single function, instead
of up to two (entry and return) in the case of FBT.
A result of this is that
.Cm dtrace -l -P kinst
will not match any probes.
.Sh IMPLEMENTATION NOTES
The provider is currently implemented only for amd64.
.Sh EXAMPLES
Find the offset corresponding to the third instruction in
.Fn vm_fault
and trace it, printing the contents of the RSI register:
.Bd -literal -offset indent
# kgdb
(kgdb) disas /r vm_fault
Dump of assembler code for function vm_fault:
   0xffffffff80876df0 <+0>:     55      push   %rbp
   0xffffffff80876df1 <+1>:     48 89 e5        mov    %rsp,%rbp
   0xffffffff80876df4 <+4>:     41 57   push   %r15

# dtrace -n 'kinst::vm_fault:4 {printf("%#x", regs[R_RSI]);}'
  2  81500                       vm_fault:4 0x827c56000
  2  81500                       vm_fault:4 0x827878000
  2  81500                       vm_fault:4 0x1fab9bef0000
  2  81500                       vm_fault:4 0xe16cf749000
  0  81500                       vm_fault:4 0x13587c366000
  ...
.Ed
.Pp
Trace all instructions in
.Fn amd64_syscall :
.Bd -literal -offset indent
# dtrace -n 'kinst::amd64_syscall:'
.Ed
.Sh SEE ALSO
.Xr dtrace 1
.Sh HISTORY
The
.Nm kinst
provider first appeared in
.Fx
14.0.
.Sh AUTHORS
This manual page was written by
.An Christos Margiolis Aq Mt christos@FreeBSD.org .