Subject: Re: [RFC PATCH v1 44/50] arm64: ptr auth: Use get_random_u64 instead of _bytes

From: George Spelvin <[email-masked]>
Date: Tue, 31 Mar 2020 00:27:38 +0000

On Mon, Mar 30, 2020 at 07:32:37PM +0000, George Spelvin wrote:
> On Mon, Mar 30, 2020 at 11:57:45AM +0100, Mark Rutland wrote:
>> As I am unaware, how does the cost of get_random_bytes() compare to the
>> cost of get_random_u64()?
> 
> It's approximately 8 times the cost.

Just a expand on on a point I may have left unclear: One 
get_random_bytes(), for a length up to 32 bytes, is approximately
8x the one get_random_u64().  (Then it jumps to 16x for up
to 96 bytes.)

Since were're using *two* get_random_u64() calls to replace one
get_random_bytes(), it's a 4x cost difference between the two
alternative ways of generating a 128-bit key.

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