On 18 September 2018 at 14:01, Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
> 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?
>
That depends on what the right thing is. 'Fastest' does not
necessarily mean 'optimal', and I guess the thermal throttling on
Skylake-X may still result in the most power efficient implementation,
which may be the preferred one in some contexts.
The point is that this is a policy decision, and those belong in
userland not in the kernel.
> 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
