RE: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

[Reshetova, Elena]

> From: Reshetova, Elena
> > Sent: 03 May 2019 17:17
> ...
> > rdrand (calling every 8 syscalls): Simple syscall: 0.0795 microseconds
> 
> You could try something like:
> 	u64 rand_val = cpu_var->syscall_rand
> 
> 	while (unlikely(rand_val == 0))
> 		rand_val = rdrand64();
> 
> 	stack_offset = rand_val & 0xff;
> 	rand_val >>= 6;
> 	if (likely(rand_val >= 4))
> 		cpu_var->syscall_rand = rand_val;
> 	else
> 		cpu_var->syscall_rand = rdrand64();
> 
> 	return stack_offset;
> 
> That gives you 10 system calls per rdrand instruction
> and mostly takes the latency out of line.

I am not really happy going the rdrand path for a couple of reasons:
- it is not available on older PCs
- its performance varies across CPUs that support it (and as I understood varies quite some)
- it is x86 centric and not generic

So, if we can use get_random_bytes() interface without tightening ourselves to
a particular instruction, I think it would be better. 
The numbers I have measured so far for buffer size of 4096 is SW only, 
I will try to measure today what boost (if any) we can have if we use SIMD 
code for it. 

Best Regards,
Elena.