dtrace: Add a "regs" variable

This allows invop-based providers (i.e., fbt and kinst) to expose the
register file of the CPU at the point where the probe fired.  It does
not work for SDT providers because their probes are implemented as plain
function calls and so don't save registers.  It's not clear what
semantics "regs" should have for them anyway.

This is akin to "uregs", which nominally provides access to the
userspace registers.  In fact, DIF already had a DIF_VAR_REGS variable
defined, it was simply unimplemented.

Usage example: print the contents of %rdi upon each call to
amd64_syscall():

    fbt::amd64_syscall:entry {printf("%x", regs[R_RDI]);}

Note that the R_* constants are defined in /usr/lib/dtrace/regs_x86.d.
Currently there are no similar definitions for non-x86 platforms.

Reviewed by:	christos
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D36799

1 files changed, 2 insertions, 0 deletions

diff --git a/cddl/contrib/opensolaris/lib/libdtrace/common/dt_open.c b/cddl/contrib/opensolaris/lib/libdtrace/common/dt_open.c
index f55d488f4d91..9621a0fcb391 100644
--- a/cddl/contrib/opensolaris/lib/libdtrace/common/dt_open.c
+++ b/cddl/contrib/opensolaris/lib/libdtrace/common/dt_open.c
@@ -416,6 +416,8 @@ static const dt_ident_t _dtrace_globals[] = {
 	&dt_idops_func, "void(int)" },
 { "rand", DT_IDENT_FUNC, 0, DIF_SUBR_RAND, DT_ATTR_STABCMN, DT_VERS_1_0,
 	&dt_idops_func, "int()" },
+{ "regs", DT_IDENT_ARRAY, 0, DIF_VAR_REGS, DT_ATTR_STABCMN, DT_VERS_1_13,
+	&dt_idops_regs, NULL },
 { "rindex", DT_IDENT_FUNC, 0, DIF_SUBR_RINDEX, DT_ATTR_STABCMN, DT_VERS_1_1,
 	&dt_idops_func, "int(const char *, const char *, [int])" },
 #ifdef illumos