On Thursday, April 25th, 2024 at 4:57 AM, Caleb Connolly caleb.connolly@linaro.org wrote:
On 25/04/2024 06:02, mwleeds@mailtundra.com wrote:
Hi Caleb,
Thanks for this interesting feedback. I saw these patches were already merged since you sent this but I added a few thoughts below anyway.
Ah, a shame.. It would be good to find a scalable solution to this!
While I agree your suggestion of doing two 4 byte reads is more scalable than the malloc() patch, the code path (zfs_nvlist_lookup_uint64) is only hit when reading the ZPool label (check_pool_label) which is a once-per-boot operation operating on some metadata, so I don't think scalability is as much a concern as it would be if we were hitting this code path when reading every block from the disk for example.
That said, I may try implementing the patch as you suggested if I get a chance. And in any case I definitely appreciate your engagement and feedback!
On Friday, April 12th, 2024 at 11:50 AM, Caleb Connolly caleb.connolly@linaro.org wrote:
Hi Phaedrus,
On 07/04/2024 03:47, mwleeds@mailtundra.com wrote:
Without this patch, when trying to boot zfs using U-Boot on a Jetson TX2 NX (which is aarch64), I get a CPU reset error like so:
"Synchronous Abort" handler, esr 0x96000021 elr: 00000000800c9000 lr : 00000000800c8ffc (reloc) elr: 00000000fff77000 lr : 00000000fff76ffc x0 : 00000000ffb40f04 x1 : 0000000000000000 x2 : 000000000000000a x3 : 0000000003100000 x4 : 0000000003100000 x5 : 0000000000000034 x6 : 00000000fff9cc6e x7 : 000000000000000f x8 : 00000000ff7f84a0 x9 : 0000000000000008 x10: 00000000ffb40f04 x11: 0000000000000006 x12: 000000000001869f x13: 0000000000000001 x14: 00000000ff7f84bc x15: 0000000000000010 x16: 0000000000002080 x17: 00000000001fffff x18: 00000000ff7fbdd8 x19: 00000000ffb405f8 x20: 00000000ffb40dd0 x21: 00000000fffabe5e x22: 000000ea77940000 x23: 00000000ffb42090 x24: 0000000000000000 x25: 0000000000000000 x26: 0000000000000000 x27: 0000000000000000 x28: 0000000000bab10c x29: 00000000ff7f85f0
Code: d00001a0 9103a000 94006ac6 f9401ba0 (f9400000) Resetting CPU ...
This happens when be64_to_cpu() is called on a value that exists at a memory address that's 4 byte aligned but not 8 byte aligned (e.g. an address ending in 04). The call stack where that happens is: check_pool_label() -> zfs_nvlist_lookup_uint64(vdevnvlist, ZPOOL_CONFIG_ASHIFT,...) -> be64_to_cpu()
This is very odd, aarch64 doesn't generally have these restrictions. I got a bit nerdsniped when I saw this so I did some digging and figured this:
- Your abort exception doesn't include the FAR_ELx register (which
should contain the address that was being accessed when the abort occured). This means your board is running in EL3. 2. It turns out there is an "A" flag in the SCTLR_ELx register, when set this flag causes a fault when trying to load from an address that isn't aligned to the size of the data element (see "A, bit" on https://developer.arm.com/documentation/ddi0595/2021-06/AArch64-Registers/SC...
I'm not sure who's in the "wrong" here, maybe the driver should avoid unaligned accesses? But then again, I don't think you should be running a ZFS driver in EL3.
I'm not familiar with the Jetson Nano, but maybe there's a documented way to run U-Boot so that it isn't executing in EL3? Or if not you could also try unsetting the A flag.
I may look into this if I get a chance. However if I write some assembly code to change the execution level or unset the A flag, I worry that the code would work fine on the hardware I have (Jetson TX2 NX) but behave differently on
Well, unsetting the A flag might arguably have some niche negative outcomes, but I really struggle to see how this is a driver bug to be honest...
another platform. And I don't think I can easily set up testing environments with u-boot + zfs on different platforms to find out.
fwiw, if you do intend to investigate this further, I'd be happy to validate your assumptions on a Qualcomm board (where we are rather boring and running in EL1 or EL2).
Thanks, I'll keep that in mind
If this really is something to fix in the driver, I don't think hotpatching every unaligned access with a malloc() is the right solution.
I'm certainly open to other ideas. The difficulty is the data structure we're parsing in this file is read from disk and it's only 4-byte aligned.
According to the ARM docs, the issue is that you're loading an 8-byte value from a 4-byte aligned address. If you split it into two 4-byte reads (for the upper and lower words) then it should work fine. This would avoid the malloc at any rate.
Makes sense
Thanks and regards,
Signed-off-by: Phaedrus Leeds mwleeds@mailtundra.com Tested-by: Phaedrus Leeds mwleeds@mailtundra.com
regarding your question about re-sending to remove these tags, I'd say probably yes, and especially if you're going to send a new revision anyway.
fwiw you seem to have gotten pretty much everything else about the patch submission process spot on :)
Kind regards,
fs/zfs/zfs.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+)
diff --git a/fs/zfs/zfs.c b/fs/zfs/zfs.c index 61d58fce68..9a50deac18 100644 --- a/fs/zfs/zfs.c +++ b/fs/zfs/zfs.c @@ -1552,35 +1552,53 @@ nvlist_find_value(char *nvlist, char *name, int valtype, char **val, if (nelm_out) *nelm_out = nelm; return 1; }
nvlist += encode_size; /* goto the next nvpair */ } return 0; }
+int is_word_aligned_ptr(void *ptr) {
- return ((uintptr_t)ptr & (sizeof(void *) - 1)) == 0;
+}
int zfs_nvlist_lookup_uint64(char *nvlist, char *name, uint64_t *out) { char *nvpair; size_t size; int found;
found = nvlist_find_value(nvlist, name, DATA_TYPE_UINT64, &nvpair, &size, 0); if (!found) return 0; if (size < sizeof(uint64_t)) { printf("invalid uint64\n"); return ZFS_ERR_BAD_FS; }
- /* On arm64, calling be64_to_cpu() on a value stored at a memory address
- that's not 8-byte aligned causes the CPU to reset. Avoid that by copying the
- value somewhere else if needed.
- */
- if (!is_word_aligned_ptr((void *)nvpair)) {
- uint64_t *alignedptr = malloc(sizeof(uint64_t));
- if (!alignedptr)
- return 0;
- memcpy(alignedptr, nvpair, sizeof(uint64_t));
- *out = be64_to_cpu(*alignedptr);
- free(alignedptr);
- return 1;
- }
out = be64_to_cpu((uint64_t *) nvpair); return 1; }
char * zfs_nvlist_lookup_string(char *nvlist, char *name) { char *nvpair; char *ret; size_t slen;
-- // Caleb (they/them)
-- // Caleb (they/them)