Re: sysfs integrity fields use

On Tue, Feb 25, 2025 at 10:23:49AM +0100, Milan Broz wrote:

Hi,

I tried to add some support for using devices with PI/DIF metadata
For an NVMe drive, reformatted to 4096 + 64 profile, I see this:

Based on everything below you have no formatted it with PI and you
also don't plan to use PI, just plain non-integrity metadata.

- /sys/block/<disk>/integrity/device_is_integrity_capable
  Contains 0 (?)
  According to docs, this field
 "Indicates whether a storage device is capable of storing integrity metadata.
 Set if the device is T10 PI-capable."

this is only 1 if protection information is enabled, and 0
for non-PI metadata.

- /sys/block/<disk>/integrity/tag_size
  Contains 0 (?)
  According to docs, this is "Number of bytes of integrity tag space
  available per 512 bytes of data."
  (I think 512 bytes is incorrect; it should be sector size, or perhaps
   value in protection_interval_bytes, though.)

Yes, it should be an integrity interval.  Which for all current drivers
is the LBA size.  And the tag_size is the size of the PI metadata, so
0 is expected.

Then we have new (undocumented) value for NVMe in
- /sys/block/<nvme>/integrity/metadata_bytes
  This contains the correct 64.

Yes, this contains the full size of the metadata.  And besides
documenting it we should probably also lift it to the block layer.

Anyway, when I try to use it (for authentication tags in dm-crypt), it works.

Should tag_size and device_is_integrity_capable be set even for the "nop" format?
Is it a bug or a feature? :-)

It is expected.  The only issue is that the block support for metadata
is called integrity all over because it was initially added for PI only
and then extended for non-PI metadata, which makes things a little
confusing.

If not, what is the correct way to read per-sector metadata size (not only for NVMe
as metadata_bytes is not available for other block devices)?

Right now the nvme specific attribute is the only way.