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sifive_e : erroneous CLINT frequency
Description of problem:
CLINT's `mtime` updates at a clock frequency of [10 MHz](https://gitlab.com/qemu-project/qemu/-/blob/master/hw/riscv/sifive_e.c?ref_type=heads#L228), whereas [SiFive documentation](https://cdn.sparkfun.com/assets/7/f/0/2/7/fe310-g002-manual-v19p05.pdf?_gl=1*w2ieef*_ga*MTcyNDI2MjM0Ny4xNjk2ODcwNTM3*_ga_T369JS7J9N*MTY5Njg3MDUzNy4xLjAuMTY5Njg3MDUzNy42MC4wLjA.) shows that its clock frequency is 32.768 kHz (i.e., the RTC clock).
This difference leads to unexpected timing behavior. Due to the difference, it can even trigger multiple nested interrupts as the IRQ routine is not able to return before a new timer interrupt is triggered.
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