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dtrace_kinst.4: mention dtrace -l -P kinst, give a more complete example

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Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D38801
This commit is contained in:
Christos Margiolis 2023-02-27 09:44:13 -05:00 committed by Mark Johnston
parent cd29688e8e
commit 2d71406a6f
1 changed files with 18 additions and 14 deletions
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32
share/man/man4/dtrace_kinst.4
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@ -24,37 +24,38 @@
.\"
.\" $FreeBSD$
.\"
.Dd October 01, 2022
.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.
.Nd a DTrace provider for tracing arbitrary instructions in a given kernel function
.Sh SYNOPSIS
kinst::<function>:<instruction>
.Sh DESCRIPTION
.Pp
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
.Xr kgdb 1 .
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
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
.Pp
Find the offset to the third instruction in
Find the offset corresponding to the third instruction in
.Fn vm_fault
and trace it:
.Bd -literal -offset indent
@ -66,6 +67,12 @@ Dump of assembler code for function vm_fault:
0xffffffff80876df4 <+4>: 41 57 push %r15
# dtrace -n 'kinst::vm_fault:4'
2 81500 vm_fault:4
2 81500 vm_fault:4
2 81500 vm_fault:4
2 81500 vm_fault:4
0 81500 vm_fault:4
...
.Ed
.Pp
Trace all instructions in
@ -73,11 +80,8 @@ Trace all instructions in
.Bd -literal -offset indent
# dtrace -n 'kinst::amd64_syscall:'
.Ed
.Sh IMPLEMENTATION NOTES
The provider is currently implemented only for amd64.
.Sh SEE ALSO
.Xr dtrace 1 ,
.Xr kgdb 1
.Xr dtrace 1
.Sh HISTORY
The
.Nm kinst