Date: Thu, 20 Sep 2012 11:58:54 +0200 From: =?utf-8?Q?Dag-Erling_Sm=C3=B8rgrav?= <des@des.no> To: Pawel Jakub Dawidek <pjd@FreeBSD.org> Cc: freebsd-security@freebsd.org, Jonathan Anderson <jonathan.anderson@cl.cam.ac.uk> Subject: Re: Collecting entropy from device_attach() times. Message-ID: <86boh1t3q9.fsf@ds4.des.no> In-Reply-To: <20120919192923.GA1416@garage.freebsd.pl> (Pawel Jakub Dawidek's message of "Wed, 19 Sep 2012 21:29:24 %2B0200") References: <20120918211422.GA1400@garage.freebsd.pl> <A8FD98DD94774D00B4E5F78D3174C1B4@gmail.com> <20120919192923.GA1416@garage.freebsd.pl>
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Pawel Jakub Dawidek <pjd@FreeBSD.org> writes: > http://people.freebsd.org/~pjd/patches/harvest_device_attach.2.patch You can replace highbit(x) - 9 with flsll(x) - 10. Unfortunately, we don't have flsll() in the kernel, but here's a simple implementation: /* * Find last bit set in an unsigned long long. Assumes that ULL is * always 64 bits wide while UL may be either 32 or 64 bits wide. */ static __inline unsigned int flsll(unsigned long long mask) { #ifdef __LP64__ return (flsl(mask)); #else return (mask >> 32 ? 32 + flsl(mask >> 32) : flsl(mask)); #endif } On i386 and amd64, flsl() is an inline function that expands to a single assembler instruction. On all other platforms, it is a function in libkern, which is stupid - gcc and clang have builtin functions for it which are almost certainly faster than a function call. Same goes for s/last bit/first bit/; s/fls/ffs/g. DES --=20 Dag-Erling Sm=C3=B8rgrav - des@des.no
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