Re: [PATCH 7/7] dm-verity: hash blocks with shash import+finup when possible

[Eric Biggers <ebiggers@kernel.org>]

Hi Ard,

On Tue, Jul 02, 2024 at 09:41:19AM +0200, Ard Biesheuvel wrote:
> >  int verity_hash(struct dm_verity *v, struct dm_verity_io *io,
> >                 const u8 *data, size_t len, u8 *digest, bool may_sleep)
> >  {
> > -       struct ahash_request *req = verity_io_hash_req(v, io);
> >         int r;
> > -       struct crypto_wait wait;
> > -
> > -       r = verity_hash_init(v, req, &wait, may_sleep);
> > -       if (unlikely(r < 0))
> > -               goto out;
> >
> > -       r = verity_hash_update(v, req, data, len, &wait);
> > -       if (unlikely(r < 0))
> > -               goto out;
> > +       if (static_branch_unlikely(&ahash_enabled) && !v->shash_tfm) {
> 
> Is the static key really worth the hassle? Couldn't this just be
> 
> if (unlikely(!v->shash_tfm)) {
> 
> so that the ahash logic is moved to the cold path? We need to
> dereference v->shash_tfm right away in any case, and if it is never
> NULL, the branch predictor should be able to remember that.

The value of the static key is indeed marginal.  I included it because of the
precedent of dm-verity's existing use_bh_wq_enabled static key, which exists for
a similar purpose.  As long as we're going through the trouble of doing that, I
think it makes sense to use the same pattern for ahash too.  It's another rarely
needed option that can be patched in in the very rare case that it's needed.

- Eric