From 6fa912f987faa03add4031822df13e4999a8cbac Mon Sep 17 00:00:00 2001 From: "Jason A. Donenfeld" Date: Fri, 8 Apr 2022 18:03:13 +0200 Subject: [PATCH] mips: use fallback for random_get_entropy() instead of just c0 random commit 1c99c6a7c3c599a68321b01b9ec243215ede5a68 upstream. For situations in which we don't have a c0 counter register available, we've been falling back to reading the c0 "random" register, which is usually bounded by the amount of TLB entries and changes every other cycle or so. This means it wraps extremely often. We can do better by combining this fast-changing counter with a potentially slower-changing counter from random_get_entropy_fallback() in the more significant bits. This commit combines the two, taking into account that the changing bits are in a different bit position depending on the CPU model. In addition, we previously were falling back to 0 for ancient CPUs that Linux does not support anyway; remove that dead path entirely. Cc: Thomas Gleixner Cc: Arnd Bergmann Tested-by: Maciej W. Rozycki Acked-by: Thomas Bogendoerfer Signed-off-by: Jason A. Donenfeld Signed-off-by: Greg Kroah-Hartman --- arch/mips/include/asm/timex.h | 17 ++++++++--------- 1 file changed, 8 insertions(+), 9 deletions(-) diff --git a/arch/mips/include/asm/timex.h b/arch/mips/include/asm/timex.h index 8026baf46e72..2e107886f97a 100644 --- a/arch/mips/include/asm/timex.h +++ b/arch/mips/include/asm/timex.h @@ -76,25 +76,24 @@ static inline cycles_t get_cycles(void) else return 0; /* no usable counter */ } +#define get_cycles get_cycles /* * Like get_cycles - but where c0_count is not available we desperately * use c0_random in an attempt to get at least a little bit of entropy. - * - * R6000 and R6000A neither have a count register nor a random register. - * That leaves no entropy source in the CPU itself. */ static inline unsigned long random_get_entropy(void) { - unsigned int prid = read_c0_prid(); - unsigned int imp = prid & PRID_IMP_MASK; + unsigned int c0_random; - if (can_use_mips_counter(prid)) + if (can_use_mips_counter(read_c0_prid())) return read_c0_count(); - else if (likely(imp != PRID_IMP_R6000 && imp != PRID_IMP_R6000A)) - return read_c0_random(); + + if (cpu_has_3kex) + c0_random = (read_c0_random() >> 8) & 0x3f; else - return 0; /* no usable register */ + c0_random = read_c0_random() & 0x3f; + return (random_get_entropy_fallback() << 6) | (0x3f - c0_random); } #define random_get_entropy random_get_entropy