Hi Ard,
On Tue, Sep 18, 2018 at 11:28:50AM -0700, Ard Biesheuvel wrote:
> On 18 September 2018 at 09:16, Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
> > - While I initially wasn't going to do this for the initial
> > patchset, it was just so simple to do: now there's a nosimd
> > module parameter that can be used to disable simd instructions
> > for debugging and testing, or on weird systems.
> >
>
> I was going to respond in the other thread but it is probably better
> to move the discussion here.
>
> My concern about the monolithic nature of each algo module is not only
> about SIMD, and it has nothing to do with weird systems. It has to do
> with micro-architectural differences which are more common on ARM than
> on other architectures *, I suppose. But generalizing from that, it
> has to do with policy which is currently owned by userland and not by
> the kernel. This will also be important for choosing between the time
> variant but less safe table based scalar AES and the much slower time
> invariant version (which is substantially slower, especially on
> decryption) once we move AES into this library.
>
> So a command line option for the kernel is not the solution here. If
> we can't have separate modules, could we at least have per-module
> options that put the policy decisions back into userland?
>
> * as an example, the SHA256 NEON code I collaborated on with Andy
> Polyakov 2 years ago is significantly faster on some cores and not on
> others
Interesting concern. There are micro-architectural quirks on x86 too
that the current code actually already considers. Notably, we use an
AVX-512VL path for Skylake-X but an AVX-512F path for Knights Landing
and Coffee Lake and others, due to thermal throttling when touching the
zmm registers on Skylake-X. So, in the code, we have it automatically
select the right thing based on the micro-architecture.
Is the same thing not possible with ARM? Do you not have access to this
information already, such that the module can just always do the right
thing and not require any user intervention?
If so, that would be ideal. If not (and I'm curious to learn why not
exactly), then indeed we could add some runtime nobs in /sys/module/
{algo}/parameters/{nob}, or the like. This would be super easy to do,
should we ever encounter a situation where we're unable to auto-detect
the correct thing.
Regards,
Jason
