timer_create, timer_settime and timer_delete syscall implementation

[ghost]

Started prototyping a bit today to add timer support and ran into two issues.

1. Can we use a global timerid counter in libos_utils.c? Or would we want a separate handle map inside libos_handle.c?
2. The syscall alarm seems to be using the _PalSystemTimeQuery OCALL implemented in pal_misc.c which only supports CLOCK_REALTIME. Can we continue ignoring other clock types? Is there an alternative clock source for the enclave?

[dimakuv]

1. Can we use a global timerid counter in libos_utils.c? Or would we want a separate handle map inside libos_handle.c?

I think you can use a global counter, instead of adding a new handle map. There are two reasons for this:

1. Simplicity. It looks like timer IDs are implemented as rather complicated numbers in Linux, but without any connections to other handles (file descriptions). So we can do something similar -- just have a global counter, for the sake of simplicity and self-containment.

2. POSIX timers are process-internal, i.e., they do not propagate over fork() and execve(). This is described in the man page:

   Timers are not inherited by the child of a fork(2), and are disarmed and deleted during an execve(2).

But I would prefer to put all POSIX-timer functionality, including this global timerid counter, in either of these:

• https://github.com/gramineproject/gramine/blob/master/libos/src/sys/libos_time.c (preferred by me)
• https://github.com/gramineproject/gramine/blob/master/libos/src/sys/libos_posix_time.c (new file, if you feel like it deserves its own file)

But definitely I don't like the idea of using libos_utils.c for POSIX timers (or parts of POSIX timers).

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2. The syscall alarm seems to be using the _PalSystemTimeQuery OCALL implemented in pal_misc.c which only supports CLOCK_REALTIME. Can we continue ignoring other clock types? Is there an alternative clock source for the enclave?

Your understanding is correct. There is no alternative clock source for the enclave.

I suggest to follow the logic as here:

gramine/libos/src/sys/libos_sleep.c

Line 76 in 0119fd2

/* In Gramine all clocks are the same. */

In other words, emulate all clocks as CLOCK_REALTIME and output a warning about this.

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Based on my current understanding adding timer support will require timer_create, timer_settime and timer_delete to be implemented.

Yes. Actually, according to the man page, you'll need to implement the following syscalls:

• timer_create(): Create a timer.

• timer_settime(2): Arm (start) or disarm (stop) a timer.

• timer_gettime(2): Fetch the time remaining until the next expiration of a timer, along with the interval setting of the timer.

• timer_getoverrun(2): Return the overrun count for the last timer expiration.

• timer_delete(2): Disarm and delete a timer.

By the way, I would recommend to start with only SIGEV_NONE and SIGEV_SIGNAL support in your first PR. The other types (SIGEV_THREAD and SIGEV_THREAD_ID) seem more complicated and may be added in a future PR (if required).

I think I already mentioned this in the past, but please use the following logic as your guide in POSIX-timers implementation:

• https://github.com/gramineproject/gramine/blob/master/libos/src/sys/libos_time.c
• https://github.com/gramineproject/gramine/blob/master/libos/src/sys/libos_sleep.c

[ghost]

Thanks for the in-depth response!