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Extended emubd to test metastability, added ckprog/ckread tests

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Metastability is a rather nasty error condition where successive reads
to a memory location may return different values, either due to bus
issues or a failed prog. It's a tricky error condition to detect, and
one that ckreads was, in theory, supposed to help with.

To help test metastability (and other single-bit errors), emubd gained
several new features:

- LFS_EMUBD_BADBLOCK_PROGFLIP    - Prog flips a bit
- LFS_EMUBD_BADBLOCK_READFLIP    - Read flips a bit sometimes
- LFS_EMUBD_POWERLOSS_METASTABLE - Reads may flip a bit

These only affect a single bit in a given block, but by randomizing
which bit during every erase (and exhaustive bit testing in test_ck) we
should still see some fairly interesting bit-error patterns over time.

It's a bit difficult to test with more than a single bit error because
you can quickly find checksum/parity collisions when fuzz testing. But
there may be other interesting error patterns to look at in the future?

Also the erase_cycles implementation got a bit of a rework since it was
lopsided previously (progs/reads would always error before erases). And
since I was messing with emubd's internals I added lfs_emubd_markbad/
markgood and a few other convenience functions that seem useful:

- lfs_emubd_seed - Manually set the prng, needed in test_ck actually
- lfs_emubd_markbad - Mark block as bad, same as wear=-1
- lfs_emubd_markgood - Mark block as good, same as wear=0
- lfs_emubd_badbit - Get which big failed
- lfs_emubd_setbadbit - Set which bit will fail
- lfs_emubd_randomizebadbit - Randomize bad bit on erase
- lfs_emubd_markbadbit - Mark bit as bad, same as setbadbit+markbad

---

The intention of this new metastability emulation was to extend test_ck
to test ckreads/ckprogs. This went... interestingly.

The good news, the new emulation and tests worked quite well. They were
able to quite quickly show that ckreads is fundamentally not able to
detect all single-bit errors in our current design.

The problem boils down to the fact that the location of our parity bits
depends on the tag's leb128-encoded size. If a bit flip changes this
size field, we end up with a new parity bit, which 50/50 may or may not
detect the error.

For example, one bit flip:

  40 0c 00 12 80 0d ff ff
  '----.----' ^--------------------.
       '- altble 0xc w0 -18 parity=1

  40 0c 80 12 80 0d ff ff
  '-------.-------' ^----------------------.
          '- altble 0xc w2304 -1664 parity=1

This doesn't make ckreads _completely_ useless, just mostly useless. We
can still use it to check parity bits, but without a systematic proof.

But there's enough problems with ckreads: performance, RAM, code, etc,
that I think it may just be an interesting proof-of-concept and not
something users should actually use. Checking reads in the bd-layer
solves all of these problems...

---

At the very least ckprogs gets better testing, thanks to new tests in
test_ck and the addition of LFS_EMUBD_BADBLOCK_PROGFLIP in
test_badblocks.

The extra testing also found a ckprog/ckread hole in that we don't
ckprog/ckread during lfsr_format! I fixed this by making lfsr_format
always use ckprogs/ckreads if available, but maybe lfsr_format should
take its own set of flags?

Funnily enough this had no impact on code size since it probably just
changed the constant in a constant pool:

          code           stack
  before: 37872           3048
  after:  37872 (+0.0%)   3048 (+0.0%)
This commit is contained in:
Christopher Haster
2024-08-10 17:41:51 -05:00
parent 89565ec513
commit 458fe16f38
5 changed files with 1174 additions and 135 deletions
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bd/lfs_emubd.h
+44 -15
View File
@@ -31,25 +31,25 @@ extern "C"
// Mode determining how "bad-blocks" behave during testing. This simulates
// some real-world circumstances such as progs not sticking (prog-noop),
// a readonly disk (erase-noop), and ECC failures (read-error).
//
// Not that read-noop is not allowed. Read _must_ return a consistent (but
// may be arbitrary) value on every read.
// a readonly disk (erase-noop), ECC failures (read-error), and of course,
// random bit failures (prog-flip, read-flip)
typedef enum lfs_emubd_badblock_behavior {
LFS_EMUBD_BADBLOCK_PROGERROR = 0, // Error on prog
LFS_EMUBD_BADBLOCK_ERASEERROR = 1, // Error on erase
LFS_EMUBD_BADBLOCK_READERROR = 2, // Error on read
LFS_EMUBD_BADBLOCK_PROGNOOP = 3, // Prog does nothing silently
LFS_EMUBD_BADBLOCK_ERASENOOP = 4, // Erase does nothing silently
LFS_EMUBD_BADBLOCK_PROGERROR = 0, // Error on prog
LFS_EMUBD_BADBLOCK_ERASEERROR = 1, // Error on erase
LFS_EMUBD_BADBLOCK_READERROR = 2, // Error on read
LFS_EMUBD_BADBLOCK_PROGNOOP = 3, // Prog does nothing silently
LFS_EMUBD_BADBLOCK_ERASENOOP = 4, // Erase does nothing silently
LFS_EMUBD_BADBLOCK_PROGFLIP = 5, // Prog flips a bit
LFS_EMUBD_BADBLOCK_READFLIP = 6, // Read flips a bit sometimes
} lfs_emubd_badblock_behavior_t;
// Mode determining how power-loss behaves during testing. For now this
// only supports a noop behavior, leaving the data on-disk untouched.
// Mode determining how power-loss behaves during testing.
typedef enum lfs_emubd_powerloss_behavior {
LFS_EMUBD_POWERLOSS_NOOP = 0, // Progs are atomic
LFS_EMUBD_POWERLOSS_SOMEBITS = 1, // One bit is progged
LFS_EMUBD_POWERLOSS_MOSTBITS = 2, // All-but-one bit is progged
LFS_EMUBD_POWERLOSS_OOO = 3, // Blocks are written out-of-order
LFS_EMUBD_POWERLOSS_NOOP = 0, // Progs are atomic
LFS_EMUBD_POWERLOSS_SOMEBITS = 1, // One bit is progged
LFS_EMUBD_POWERLOSS_MOSTBITS = 2, // All-but-one bit is progged
LFS_EMUBD_POWERLOSS_OOO = 3, // Blocks are written out-of-order
LFS_EMUBD_POWERLOSS_METASTABLE = 4, // Reads may flip a bit
} lfs_emubd_powerloss_behavior_t;
// Type for measuring read/program/erase operations
@@ -121,6 +121,10 @@ struct lfs_emubd_config {
typedef struct lfs_emubd_block {
uint32_t rc;
lfs_emubd_wear_t wear;
bool metastable;
// sign(bad_bit)=0 => randomized on erase
// sign(bad_bit)=1 => fixed
lfs_size_t bad_bit;
uint8_t data[];
} lfs_emubd_block_t;
@@ -186,6 +190,9 @@ int lfs_emubd_sync(const struct lfs_config *cfg);
/// Additional extended API for driving test features ///
// Set the current prng state
int lfs_emubd_seed(const struct lfs_config *cfg, uint32_t seed);
// A checksum of a block for debugging purposes
int lfs_emubd_cksum(const struct lfs_config *cfg,
lfs_block_t block, uint32_t *cksum);
@@ -219,6 +226,28 @@ lfs_emubd_swear_t lfs_emubd_wear(const struct lfs_config *cfg,
int lfs_emubd_setwear(const struct lfs_config *cfg,
lfs_block_t block, lfs_emubd_wear_t wear);
// Mark a block as bad, this is equivalent to setting wear to maximum
int lfs_emubd_markbad(const struct lfs_config *cfg, lfs_block_t block);
// Clear any simulated wear on a given block
int lfs_emubd_markgood(const struct lfs_config *cfg, lfs_block_t block);
// Get which bit failed, this changes on erase/power-loss unless manually set
lfs_ssize_t lfs_emubd_badbit(const struct lfs_config *cfg,
lfs_block_t block);
// Set which bit should fail in a given block
int lfs_emubd_setbadbit(const struct lfs_config *cfg,
lfs_block_t block, lfs_size_t bit);
// Randomize the bad bit on erase (the default)
int lfs_emubd_randomizebadbit(const struct lfs_config *cfg,
lfs_block_t block);
// Mark a block as bad and which bit should fail
int lfs_emubd_markbadbit(const struct lfs_config *cfg,
lfs_block_t block, lfs_size_t bit);
// Get the remaining power-cycles
lfs_emubd_spowercycles_t lfs_emubd_powercycles(
const struct lfs_config *cfg);