Re: [PATCH v4 02/16] xmon: Move out-of-line instructions to text section

On Mon, Mar 23, 2020 at 5:05 PM Balamuruhan S [off-list ref] wrote:

On Fri, 2020-03-20 at 16:17 +1100, Jordan Niethe wrote:

quoted

To execute an instruction out of line after a breakpoint, the NIP is
set
to the address of struct bpt::instr. Here a copy of the instruction
that
was replaced with a breakpoint is kept, along with a trap so normal
flow
can be resumed after XOLing. The struct bpt's are located within the
data section. This is problematic as the data section may be marked
as
no execute.

Instead of each struct bpt holding the instructions to be XOL'd, make
a
new array, bpt_table[], with enough space to hold instructions for
the
number of supported breakpoints. Place this array in the text
section.
Make struct bpt::instr a pointer to the instructions in bpt_table[]
associated with that breakpoint. This association is a simple
mapping:
bpts[n] -> bpt_table[n * words per breakpoint].

Can we have it in separate commits ?
        * introduce the array bpt_table[] and make struct bpt::instr a
                pointer to the instructions in bpt_table[].
        * place the array in text section.

Yeah I can split it if that would be clearer.

quoted

Currently we only need
the copied instruction followed by a trap, so 2 words per breakpoint.

Signed-off-by: Jordan Niethe <redacted>
---
v4: New to series
---
 arch/powerpc/kernel/vmlinux.lds.S |  2 +-
 arch/powerpc/xmon/xmon.c          | 22 +++++++++++++---------
 2 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/arch/powerpc/kernel/vmlinux.lds.S

b/arch/powerpc/kernel/vmlinux.lds.S
index b4c89a1acebb..e90845b8c300 100644

--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S

@@ -86,7 +86,7 @@ SECTIONS
              ALIGN_FUNCTION();
 #endif
              /* careful! __ftr_alt_* sections need to be close to

.text */
-             *(.text.hot TEXT_MAIN .text.fixup .text.unlikely .fixup
__ftr_alt_* .ref.text);
+             *(.text.hot TEXT_MAIN .text.fixup .text.unlikely .fixup
__ftr_alt_* .ref.text .text.xmon_bpts);
 #ifdef CONFIG_PPC64
              *(.tramp.ftrace.text);
 #endif

diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index 02e3bd62cab4..7875d1a37770 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c

@@ -97,7 +97,7 @@ static long *xmon_fault_jmp[NR_CPUS];
 /* Breakpoint stuff */
 struct bpt {
      unsigned long   address;
-     unsigned int    instr[2];
+     unsigned int    *instr;
      atomic_t        ref_count;
      int             enabled;
      unsigned long   pad;

@@ -109,6 +109,7 @@ struct bpt {
 #define BP_DABR              4

 #define NBPTS        256
+#define BPT_WORDS    2
 static struct bpt bpts[NBPTS];
 static struct bpt dabr;
 static struct bpt *iabr;

@@ -116,6 +117,8 @@ static unsigned bpinstr = 0x7fe00008;     /* trap

*/

 #define BP_NUM(bp)   ((bp) - bpts + 1)

+static unsigned int __section(.text.xmon_bpts) bpt_table[NBPTS *
BPT_WORDS];
+
 /* Prototypes */
 static int cmds(struct pt_regs *);
 static int mread(unsigned long, void *, int);

@@ -852,16 +855,16 @@ static struct bpt *at_breakpoint(unsigned long

pc)
 static struct bpt *in_breakpoint_table(unsigned long nip, unsigned
long *offp)
 {
      unsigned long off;
+     unsigned long bp_off;

-     off = nip - (unsigned long) bpts;
-     if (off >= sizeof(bpts))
+     off = nip - (unsigned long) bpt_table;
+     if (off >= sizeof(bpt_table))
              return NULL;
-     off %= sizeof(struct bpt);
-     if (off != offsetof(struct bpt, instr[0])
-         && off != offsetof(struct bpt, instr[1]))
+     bp_off = off % (sizeof(unsigned int) * BPT_WORDS);
+     if (bp_off != 0 && bp_off != 4)
              return NULL;
-     *offp = off - offsetof(struct bpt, instr[0]);
-     return (struct bpt *) (nip - off);
+     *offp = bp_off;
+     return bpts + ((off - bp_off) / (sizeof(unsigned int) *
BPT_WORDS));

`(off - bp_off) / (sizeof(unsigned int) * BPT_WORDS)` seems to be the
actual breakpoint offset. Can we have something like,

#define NBPTS  256
#define BPT_WORDS      2
#define BPT_WORDS_SIZE (sizeof(unsigned int) * BPT_WORDS)
#define BPT_OFFSET(off, bp_word_off) ((off - bp_word_off) / \                                   BPT_WORDS_SIZE)
;
:::
:::
:::
bp_word_off = off % BPT_WORDS_SIZE;
if (bp_word_off != 0 && bp_word_off != 4)
        return NULL;
*offp = bp_word_off;
return bpts + BPT_OFFSET(off, bp_word_off);

I do agree this is not very clear int terms of the calculations I
don't really want to introduce some macros to be used just once. I
will try to think of a way to make the calculations look more clear.

-- Bala