Hi folks, The RNG uses a function called random_get_entropy() basically anytime that it needs to timestamp an event. For example, an interrupt comes in, and we mix a random_get_entropy() into the entropy pool somehow. Somebody mashes their keyboard or moves their mouse around? We mix a random_get_entropy() into the entropy pool. It's one of the main varieties of input. Unfortunately, it's always 0 on a few platforms. The RNG has accumulated various hacks to deal with this, but in general it's not great. Surely we can do better than 0. In fact, *anything* that's not the same exact value all the time would be better than 0. Even a counter that increments once per hour would be better than 0! I think you get the idea. On most platforms, random_get_entropy() is aliased to get_cycles(), which makes sense for platforms where get_cycles() is defined. RDTSC, for example, has all the characteristics we care about for this function: it's fast to acquire (i.e. acceptable in an irq handler), pretty high precision, available, forms a 2-monotone distribution, etc. But for platforms without that, what is the next best thing? Sometimes the next best thing is architecture-defined. For example, really old MIPS has the CP0 random register, which isn't a cycle counter, but is at least something. However, some platforms don't even have an architecture-defined fallback. Fortunately, the timekeeping subsystem has already solved this problem of trying to determine what the least bad clock is on constrained systems, falling back to jiffies in the worst case. By exporting the raw clock, we can get a decent fallback function for when there's no cycle counter or architecture-specific function. This series makes the RNG more useful on: m68k, RISC-V, MIPS, ARM32, NIOS II, SPARC32, Xtensa, and Usermode Linux. Previously these platforms would, in certain circumstances, but out of luck with regards to having any type of event timestamping source in the RNG. Finally, note that this series isn't about "jitter entropy" or other ways of initializing the RNG. That's a different topic for a different thread. Please don't let this discussion veer off into that. Here, I'm just trying to find a good fallback counter/timer for platforms without get_cycles(), a question with limited scope. If this (or a future revision) looks good to you all and receives the requisite acks, my plan was to take these through the random.git tree for 5.19, so that I can then build on top of it. Alternatively, maybe Thomas wants to take it through his timekeeping tree? Or something else. If anybody has strong preferences, please pipe up. Thanks, Jason Changes v2->v3: - Name the fallback function random_get_entropy_fallback(), so that it can be changed out as needed. - Include header with prototype in timekeeping.c to avoid compiler warning. - Export fallback function symbol. Changes v1->v2: - Use ktime_read_raw_clock() instead of sched_clock(), per Thomas' suggestion. - Drop arm64 change. - Cleanup header inclusion ordering problem. Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Arnd Bergmann <arnd@xxxxxxxx> Cc: Theodore Ts'o <tytso@xxxxxxx> Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx> Cc: Russell King <linux@xxxxxxxxxxxxxxx> Cc: Catalin Marinas <catalin.marinas@xxxxxxx> Cc: Will Deacon <will@xxxxxxxxxx> Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> Cc: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx> Cc: Paul Walmsley <paul.walmsley@xxxxxxxxxx> Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx> Cc: Albert Ou <aou@xxxxxxxxxxxxxxxxx> Cc: David S. Miller <davem@xxxxxxxxxxxxx> Cc: Richard Weinberger <richard@xxxxxx> Cc: Anton Ivanov <anton.ivanov@xxxxxxxxxxxxxxxxxx> Cc: Johannes Berg <johannes@xxxxxxxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Cc: H. Peter Anvin <hpa@xxxxxxxxx> Cc: Chris Zankel <chris@xxxxxxxxxx> Cc: Max Filippov <jcmvbkbc@xxxxxxxxx> Cc: John Stultz <john.stultz@xxxxxxxxxx> Cc: Stephen Boyd <sboyd@xxxxxxxxxx> Cc: Dinh Nguyen <dinguyen@xxxxxxxxxx> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx Cc: linux-m68k@xxxxxxxxxxxxxxxxxxxx Cc: linux-mips@xxxxxxxxxxxxxxx Cc: linux-riscv@xxxxxxxxxxxxxxxxxxx Cc: sparclinux@xxxxxxxxxxxxxxx Cc: linux-um@xxxxxxxxxxxxxxxxxxx Cc: x86@xxxxxxxxxx Cc: linux-xtensa@xxxxxxxxxxxxxxxx Jason A. Donenfeld (10): timekeeping: add raw clock fallback for random_get_entropy() m68k: use fallback for random_get_entropy() instead of zero riscv: use fallback for random_get_entropy() instead of zero mips: use fallback for random_get_entropy() instead of zero arm: use fallback for random_get_entropy() instead of zero nios2: use fallback for random_get_entropy() instead of zero x86: use fallback for random_get_entropy() instead of zero um: use fallback for random_get_entropy() instead of zero sparc: use fallback for random_get_entropy() instead of zero xtensa: use fallback for random_get_entropy() instead of zero arch/arm/include/asm/timex.h | 1 + arch/m68k/include/asm/timex.h | 2 +- arch/mips/include/asm/timex.h | 2 +- arch/nios2/include/asm/timex.h | 2 ++ arch/riscv/include/asm/timex.h | 2 +- arch/sparc/include/asm/timex_32.h | 4 +--- arch/um/include/asm/timex.h | 9 ++------- arch/x86/include/asm/tsc.h | 10 ++++++++++ arch/xtensa/include/asm/timex.h | 6 ++---- include/linux/timex.h | 8 ++++++++ kernel/time/timekeeping.c | 10 ++++++++++ 11 files changed, 39 insertions(+), 17 deletions(-) -- 2.35.1
