Re: [PATCH riscv/for-next] crypto: riscv - parallelize AES-CBC decryption

Palmer Dabbelt
2024-03-20T01:49:08Z

On Sat, 10 Feb 2024 10:12:40 PST (-0800), ebiggers@kernel.org wrote:
> On Sat, Feb 10, 2024 at 11:25:27PM +0800, Jerry Shih wrote:
>> > .macro aes_cbc_decrypt keylen
>> > + srli LEN, LEN, 2 // Convert LEN from bytes to words
>> > vle32.v v16, (IVP) // Load IV
>> > 1:
>> > - vle32.v v17, (INP) // Load ciphertext block
>> > - vmv.v.v v18, v17 // Save ciphertext block
>> > - aes_decrypt v17, \keylen // Decrypt
>> > - vxor.vv v17, v17, v16 // XOR with IV or prev ciphertext block
>> > - vse32.v v17, (OUTP) // Store plaintext block
>> > - vmv.v.v v16, v18 // Next "IV" is prev ciphertext block
>> > - addi INP, INP, 16
>> > - addi OUTP, OUTP, 16
>> > - addi LEN, LEN, -16
>> > + vsetvli t0, LEN, e32, m4, ta, ma
>> > + vle32.v v20, (INP) // Load ciphertext blocks
>> > + vslideup.vi v16, v20, 4 // Setup prev ciphertext blocks
>> > + addi t1, t0, -4
>> > + vslidedown.vx v24, v20, t1 // Save last ciphertext block
>>
>> Do we need to setup the `e32, len=t0` for next IV?
>> I think we only need 128bit IV (with VL=4).
>>
>> > + aes_decrypt v20, \keylen // Decrypt the blocks
>> > + vxor.vv v20, v20, v16 // XOR with prev ciphertext blocks
>> > + vse32.v v20, (OUTP) // Store plaintext blocks
>> > + vmv.v.v v16, v24 // Next "IV" is last ciphertext block
>>
>> Same VL issue here.
>
> It's true that the vslidedown.vx and vmv.v.v only need vl=4. But it also works
> fine with vl unchanged. It just results in some extra data being moved in the
> registers. My hypothesis is that this is going to be faster than having the
> three extra instructions per loop iteration to change the vl to 4 twice.
>
> I still have no real hardware to test on, so I have no quantitative data. All I
> can do is go with my instinct which is that the shorter version will be better.
>
> If you have access to a real CPU that supports the RISC-V vector crypto
> extensions, I'd be interested in the performance you get from each variant.
> (Of course, different RISC-V CPU implementations may have quite different
> performance characteristics, so that still won't be definitive.)

We're stacking up a lot of stuff with HW-dependent performance
questions, I think it's fine to just take what's reasonably simple for
now. If we try to speculate about what future hardware might do we're
just going to go crazy with possibilities, IMO we're way better off just
optimizing as things show up.

> Here is the alternative variant given as a diff from this patch:
>
> diff --git a/arch/riscv/crypto/aes-riscv64-zvkned.S b/arch/riscv/crypto/aes-riscv64-zvkned.S
> index 43541aad6386c..ef380771f606a 100644
> --- a/arch/riscv/crypto/aes-riscv64-zvkned.S
> +++ b/arch/riscv/crypto/aes-riscv64-zvkned.S
> @@ -146,10 +146,13 @@ SYM_FUNC_END(aes_ecb_decrypt_zvkned)
> vle32.v v20, (INP) // Load ciphertext blocks
> vslideup.vi v16, v20, 4 // Setup prev ciphertext blocks
> addi t1, t0, -4
> + vsetivli zero, 4, e32, m4, ta, ma
> vslidedown.vx v24, v20, t1 // Save last ciphertext block
> + vsetvli t0, LEN, e32, m4, ta, ma
> aes_decrypt v20, \keylen // Decrypt the blocks
> vxor.vv v20, v20, v16 // XOR with prev ciphertext blocks
> vse32.v v20, (OUTP) // Store plaintext blocks
> + vsetivli zero, 4, e32, m4, ta, ma
> vmv.v.v v16, v24 // Next "IV" is last ciphertext block
> slli t1, t0, 2 // Words to bytes
> add INP, INP, t1
> @@ -157,7 +160,6 @@ SYM_FUNC_END(aes_ecb_decrypt_zvkned)
> sub LEN, LEN, t0
> bnez LEN, 1b
>
> - vsetivli zero, 4, e32, m1, ta, ma
> vse32.v v16, (IVP) // Store next IV
> ret
> .endm
>
> A third variant would be to just replace vmv.v.v with vmv1r.v.
>
> In general, this level of micro-optimization probably needs to be wait until
> there are a variety of CPUs to test on. We know that parallelizing the
> algorithms is helpful, so we should do that, as this patch does. But the
> effects of small variations in the instruction sequences are currently unclear.

Ya, I agree. So I'm fine with this, it's a base and we can always
improve it when there's something concrete to run on.

> - Eric