Messages in this thread | | | Subject | [RFC] PATCH 0/4 - Time virtualization | Date | Thu, 13 Apr 2006 13:19:36 -0400 | From | Jeff Dike <> |
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This set of patches implements time virtualization by creating a time namespace an interface to it through unshare a ptrace extension to allow UML to take advantage of this UML support
The guts of the namespace is just an offset from the system time. Within the container, gettimeofday adds this offset to the system time. settimeofday changes the offset without touching the system time. As such, within a namespace, settimeofday is unprivileged.
The interface to it is through unshare(CLONE_TIME). This creates the new namespace, initialized with a zero offset from the system time.
The advantage of this for UML is that it can create a time namespace for itself and subsequently let its process' gettimeofday run on the host, without being intercepted and run inside UML. As such, it should basically run at native speed.
In order to allow this, we need selective system call interception. The third patch implements PTRACE_SYSCALL_MASK, which specifies, through a bitmask, which system calls are intercepted and which aren't.
Finally, the UML support is straightforward. It calls unshare(CLONE_TIME) to create the new namespace, sets gettimeofday to run without being intercepted, and makes settimeofday call the host's settimeofday instead of maintaining the time offset itself.
As expected, a gettimeofday loop runs basically at native speed. The two quick tests I did had it running inside UML at 98.8 and 99.2 % of native.
BUG - as I was writing this, I realized that refcounting of the time_ns structures is wrong - they need to be incremented at process creation and decremented at process exit.
Jeff
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