Re: [PATCH 12/12] RISC-V: crypto: add Zvkb accelerated ChaCha20 implementation

[Jerry Shih]

On Nov 21, 2023, at 21:14, Conor Dooley <conor.dooley@xxxxxxxxxxxxx> wrote:
> On Tue, Nov 21, 2023 at 06:55:07PM +0800, Jerry Shih wrote:
>> Sorry, I just use my `internal` qemu with vector-crypto and rva22 patches.
>> 
>> The public qemu haven't supported rva22 profiles. Here is the qemu patch[1] for
>> that. But here is the discussion why the qemu doesn't export these
>> `named extensions`(e.g. Zicclsm).
>> I try to add Zicclsm in DT in the v2 patch set. Maybe we will have more discussion
>> about the rva22 profiles in kernel DT.
> 
> Please do, that'll be fun! Please take some time to read what the
> profiles spec actually defines Zicclsm fore before you send those patches
> though. I think you might come to find you have misunderstood what it
> means - certainly I did the first time I saw it!

From the rva22 profile:
  This requires misaligned support for all regular load and store instructions (including
  scalar and ``vector``)

The spec includes the explicit `vector` keyword.
So, I still think we could use Zicclsm checking for these vector-crypto implementations.

My proposed patch is just a simple patch which only update the DT document and
update the isa string parser for Zicclsm. If it's still not recommend to use Zicclsm
checking, I will turn to use `RISCV_HWPROBE_MISALIGNED_*` instead.

>> [1]
>> LINK: https://lore.kernel.org/all/d1d6f2dc-55b2-4dce-a48a-4afbbf6df526@xxxxxxxxxxxxxxxx/#t
>> 
>> I don't know whether it's a good practice to check unaligned access using
>> `Zicclsm`. 
>> 
>> Here is another related cpu feature for unaligned access:
>> RISCV_HWPROBE_MISALIGNED_*
>> But it looks like it always be initialized with `RISCV_HWPROBE_MISALIGNED_SLOW`[2].
>> It implies that linux kernel always supports unaligned access. But we have the
>> actual HW which doesn't support unaligned access for vector unit.
> 
> https://docs.kernel.org/arch/riscv/uabi.html#misaligned-accesses
> 
> Misaligned accesses are part of the user ABI & the hwprobe stuff for
> that allows userspace to figure out whether they're fast (likely
> implemented in hardware), slow (likely emulated in firmware) or emulated
> in the kernel.

The HWPROBE_MISALIGNED_* checking function is at:
https://github.com/torvalds/linux/blob/c2d5304e6c648ebcf653bace7e51e0e6742e46c8/arch/riscv/kernel/cpufeature.c#L564-L647
The tests are all scalar. No `vector` test inside. So, I'm not sure the
HWPROBE_MISALIGNED_* is related to vector unit or not.

The goal is to check whether `vector` support unaligned access or not
in this crypto patch.

I haven't seen the emulated path for unaligned-vector-access in OpenSBI
and kernel. Is the unaligned-vector-access included in user ABI?

Thanks,
Jerry