On Tue, Sep 01, 2026 at 04:22:33PM +0800, Linlin Zhang wrote: > With this design, I believe there would be two VM exits associated with > encrypted I/O: > 1. Key programming > Before encrypted I/O can be submitted, the guest needs to program a key > into a virtual keyslot: > > Guest virtio-blk driver > -> VM exit > -> virtio-blk backend (e.g. QEMU) > -> ioctl > -> host virtio-blk proxy driver > (stores the blk_crypto_key in a virtual keyslot) > > 2. Encrypted I/O submission > The I/O request carries the virtual keyslot number and DUN: > > Guest virtio-blk driver > -> VM exit > -> virtio-blk backend (e.g. QEMU) > -> ioctl > -> host virtio-blk proxy driver > (looks up the blk_crypto_key associated with the virtual keyslot > and submits I/O using bio_crypt_set_ctx())
Most I/O requests only need step (2), since they reuse step (1) from a previous I/O request. The kernel evicts a keyslot only when it is the least recently used among all the keyslots and another one is needed. (Or when eviction is explicitly requested.) This is especially relevant when only a small number of keys is used, like is the case when the per-file key support in fscrypt is disabled. Programming keys on Qualcomm ICE has always been very slow even on physical hardware, and the kernel was already designed to mitigate that. > I have been wondering whether a model similar to the passthrough > blk-crypto-profile used by certain dm targets could be applicable here. > > In such a design, ownership of keyslot management would effectively and > totally > move to the host. The guest would no longer manage virtual keyslots, and the > host block layer would continue using its existing keyslot manager to > allocate, > reuse, and evict physical keyslots as needed. I think the virtual keyslots are still useful so that the key bytes, key size, key type, algorithm, and data unit size don't have to be transmitted in every I/O request, then all revalidated and processed again. There is a reason that inline encryption hardware uses keyslots, and I think the same largely applies to virtio-blk. - Eric
