Re: Cryptsetup and hardware accelerated AES-XTS

[Eric Biggers <ebiggers@kernel.org>]

On Sun, Jul 21, 2024 at 09:14:02AM +0000, Maxim Fomin wrote:
> 
> On Saturday, July 20th, 2024 at 6:36 PM, Eric Biggers <ebiggers@kernel.org> wrote:
> 
> > 
> > 
> > On Sat, Jul 20, 2024 at 12:15:23PM +0000, Maxim Fomin wrote:
> > 
> > > Hi!
> > > 
> > > Recently linux kernel got[1] faster AES-XTS on modern x86_64 CPUs thanks to VAES and AVX-10/512. I decided to dig deeper into this issue and found the article[2] from 2020 stating that dm-crypt can be configured to use faster (synchronous and hardware accelerated) algorithms with 'capi:' prefix. Can cryptsetup be configured to ask dm-crypt to use hardware accelerated algorithms?
> > > 
> > > [1] https://lore.kernel.org/lkml/20240403004404.GC2576@sol.localdomain/T/#m83293b2699f9a5da04fc5780ee402191dace3926
> > > 
> > > [2] https://blog.cloudflare.com/speeding-up-linux-disk-encryption/
> > > 
> > > Best regards,
> > > Maxim
> > 
> > 
> > You don't need to use "capi:". Just make sure CONFIG_CRYPTO_AES_NI_INTEL=y is
> > enabled in your kernel (which it already should have been since it was needed
> > for AES-NI acceleration before), and the new code will be used automatically if
> > your CPU supports it.
> > 
> > - Eric
> 
> I have heard that on some Intel CPUs AVX-512 is ignored (although it is
> supported) because of downclocking. I have rocketlake i5-11600K. How I can
> check which algorithm is actually used? How I can force cryptsetup/dm-crypt to
> enable AVX-512 version if it is not used?
> 

The way to tell which AES-XTS implementation is used by default on a particular
system is to check /proc/crypto for which "xts(aes)" has the highest priority.
There is a way to change algorithm priorities to override the default, but there
isn't much reason to think that people would be able to make a better choice
than the kernel's default.  The implementation that uses 512-bit vectors
(xts-aes-vaes-avx10_512) is only deprioritized on Ice Lake and Tiger Lake, so it
won't happen on your system anyway as it uses Rocket Lake.  On CPUs where the
deprioritization of xts-aes-vaes-avx10_512 does happen, there is a reason for
it, and it is only 512-bit vectors that are disabled, *not* AVX-512 entirely.

- Eric