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From:   David Laight <David.Laight@ACULAB.COM>
To:     "'Reshetova, Elena'" <elena.reshetova@intel.com>,
        Ingo Molnar <mingo@kernel.org>
CC:     Andy Lutomirski <luto@kernel.org>, Theodore Ts'o <tytso@mit.edu>,
        "Eric Biggers" <ebiggers3@gmail.com>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: RE: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall
Thread-Topic: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall
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Date:   Fri, 3 May 2019 16:40:53 +0000
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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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