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

[David Laight]

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.

	David

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