Encrypted salt-sector initialization vector (ESSIV) is an IV
generation method that is used in some cases by fscrypt and/or
dm-crypt. It uses the hash of the block encryption key as the
symmetric key for a block encryption pass applied to the input
IV, making low entropy IV sources more suitable for block
encryption.

This driver implements a crypto API template that can be
instantiated either as an skcipher or as an AEAD (depending on the
type of the first template argument), and which defers encryption
and decryption requests to the encapsulated cipher after applying
ESSIV to the input IV. Note that in the AEAD case, it is assumed
that the keys are presented in the same format used by the authenc
template, and that the IV appears at the end of the authenticated
associated data (AAD) region (which is how dm-crypt uses it.)

Note that the use of ESSIV is not recommended for new deployments,
and so this only needs to be enabled when interoperability with
existing encrypted volumes of filesystems is required, or when
building for a particular system that requires it (e.g., when
the SoC in question has accelerated CBC but not XTS, making CBC
combined with ESSIV the only feasible mode for h/w accelerated
block encryption)

source code:

crypto/essiv.c