Re: [PATCH RFC v2] random: implement getrandom() in vDSO

[Jason A. Donenfeld]

Hi Florian,

On Mon, Aug 01, 2022 at 02:49:17PM +0200, Jason A. Donenfeld wrote:
> What I understand you to mean is that *instead of* doing vDSO, we could
> just batch in the kernel, and reap most of the performance benefits. If
> that turns out to be true, and then we don't even need this vDSO stuff,
> I'd be really happy. So I'll give this a try.
> 
> One question is where to store that batch. On the surface, per-cpu seems
> appealing, like what we do for get_random_u32() and such for kernel
> callers. But per-cpu means disabling preemption, which then becomes a
> problem when copying into userspace, where the copies can fault. So
> maybe something more sensible is, like above, just doing this per-task.
> I'll give it a stab and will let you know what it looks like.

So doing the batching in the kernel gives roughly a 2x performance boost
for the u32 case. Below is a little hacky patch you can play with. This
isn't the face melting 15x of the vDSO approach, but it is something, I
guess.

Jason