CODEWITHSUNDEEP
riscvxv6
maksadbek
maksadbek

Reputation: 1592

How to implement sleep utility in RISC-V?

I want to implement sleep utility that receives number of seconds as an input and pauses for given seconds on a educatational xv6 operation system that runs on risc-v processors.

The OS already have system call that get number of ticks and pauses: https://github.com/mit-pdos/xv6-riscv/blob/riscv/kernel/sysproc.c#L56

Timers are initialized using a timer vector: https://github.com/mit-pdos/xv6-riscv/blob/riscv/kernel/kernelvec.S#L93 The timer vector is initialized with CLINT_MTIMECMP function that tells timer controller when to wake the next interrupt.

What I do not understand is how to know the time between the ticks and how many ticks are done during 1 second.

Upvotes: 0

Views: 2202

Answers (1)

TheThirdOne
TheThirdOne

Reputation: 427

Edit: A quick google of "qemu timebase riscv mtime" found a google groups chat which states that RDTIME is nanoseconds since boot and mtime is an emulated 10Mhz clock.

I haven't done a search to find the information you need, but I think I have some contextual information that would help you find it. I would recommend searching QEMU documentation / code (probably from Github search)for how mtime and mtimecmp work.

In section 10.1 (Counter - Base Counter and Timers) of specification1, it is explained that the RDTIME psuedo-instruction should have some fixed tick rate that can be determined based on the implementation 2. That tick rate would also be shared for mtimecmp and mtime as defined in the privileged specification 3.

I would presume the ticks used be the sleep system call would be the same as these ticks from the specifications. In that case, xv6 is just a kernel and wouldn't then define how many ticks/second there are. It seems that xv6 is made to run on top of qemu so the definition of ticks/second should be defined somewhere in the qemu code and might be documented.

From the old wiki for QEMU-riscv it should be clear that the SiFive CLINT defines the features xv6 needs to work, but I doubt that it specifies how to know the tickrate. Spike also supports the CLINT interface so it may also be instructive to search for the code in spike that handles it.

1 I used version 20191213 of the unprivileged specification as a reference

2

The RDTIME pseudoinstruction reads the low XLEN bits of the time CSR, which counts wall-clock real time that has passed from an arbitrary start time in the past. RDTIMEH is an RV32I-only in- struction that reads bits 63–32 of the same real-time counter. The underlying 64-bit counter should never overflow in practice. The execution environment should provide a means of determining the period of the real-time counter (seconds/tick). The period must be constant. The real-time clocks of all harts in a single user application should be synchronized to within one tick of the real-time clock. The environment should provide a means to determine the accuracy of the clock.

3

3.1.10 Machine Timer Registers (mtime and mtimecmp) Platforms provide a real-time counter, exposed as a memory-mapped machine-mode read-write register, mtime. mtime must run at constant frequency, and the platform must provide a mechanism for determining the timebase of mtime.

Upvotes: 1

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