On Sep 20, 2018, at 9:30 PM, Ard Biesheuvel [off-list ref] wrote:

quoted

On 20 September 2018 at 21:15, Jason A. Donenfeld [off-list ref] wrote:
Hi Andy,

quoted

On Fri, Sep 21, 2018 at 5:23 AM Andy Lutomirski [off-list ref] wrote:
At the risk on suggesting something awful: on x86_64, since we turn preemption off for simd, it wouldn’t be *completely* insane to do the crypto on the irq stack. It would look like:

kernel_fpu_call(func, arg);

And this helper would disable preemption, enable FPU, switch to the irq stack, call func(arg), disable FPU, enable preemption, and return. And we can have large IRQ stacks.

I refuse to touch this with a ten-foot pole until the lazy FPU restore patches land.

Haha. That's fun, and maybe we'll do that at some point, but I have
some other reasons too for being on a workqueue now.

Kernel mode crypto is callable from any context, and SIMD can be used
in softirq context on arm64 (and on x86, even from hardirq context
IIRC if the interrupt is taken from userland), in which case we'd
already be on the irq stack.

The x86_64 irq stack handles nesting already.