Hi Denis, On 03.03.2017 08:35, Denis Huber wrote: > Hallo Christian, and hallo Genode community. > > Preliminary: I want to read the registers of a thread A from a task from > another thread B on another task. I am using Genode 16.08 on Fiasco.OC. > I am also using the Genode mechanism of Cpu_thread::pause() before > Cpu_thread::state(), but do not receive the any register values beside > zeros. Christina told me that this problem comes from thread A being in > a syscall during the pausing. His suggestion was to extend > l4_thread_ex_regs_ret() to return the missing registers from the UTCB in > a non-blocking fashion. > > I am trying to do this, but I do not understand the syscall mechanism > fully. Am I correct with the following understanding: > > * Thread A is in a syscall: It is waiting for the answer of its IPC call. > * The answer will be stored in the UTCB. > * While the thread is waiting, another thread B tries to pause thread A. > * The pause mechanism wants to put thread A into an artifical-exception > state and additionally writes the register data to the UTCB of thread A. > * The pause and syscall mechanism both need the UTCB for themselves. > * To guarantee consistency of the UTCB, the pause method is not allowed > to be executed while thread A is in a syscall. Otherwise the syscall > could overrite the pause data (i.e. register values), if it > spontaneously returns.
And there would also be the problem of how to restart an interrupted syscall in thread A. > Assumed my understanding is correct (please tell me, if it is not), I > have to wait (i.e. block execution) until the syscall finishes and the > pause mechanism can write the register data to the UTCB. Christian > wrote, it is possible to return the missing registers from the UTCB > without blocking until the syscall finishes. How is it possible, if the > register data is not in the UTCB? The general idea is that when thread B calls l4_thread_ex_regs_ret() to find out the register state of thread A, the kernel could copy the complete register state of thread A into the UTCB of thread B. So, when l4_thread_ex_regs_ret() returns, thread B would find the complete register state of thread A in its own UTCB. Currently, only the flags, ip and sp are copied into the UTCB of thread B. So, when extending the implementation of the l4_thread_ex_regs_ret() syscall, you would need to find out where the kernel has stored the register state of thread A and then copy it completely into the UTCB of thread B and then, when l4_thread_ex_regs_ret() returns, read the additional registers from the UTCB of thread B. On the user side this could probably look something like: ... ip = ~0UL; sp = ~0UL; flags = L4_THREAD_EX_REGS_TRIGGER_EXCEPTION; l4_thread_ex_regs_ret(kcap, &ip, &sp, &flags); l4_utcb_t *utcb = l4_utcb(); r0 = utcb->mr[3]; r1 = utcb->mr[4]; ... where, according to the implementation of l4_thread_ex_regs_ret_u() in 'contrib/foc-xxx/src/kernel/foc/l4/pkg/l4sys/include/thread.h', mr[0], mr[1] and mr[2] are currently used to transfer the flags, ip and sp. Christian ------------------------------------------------------------------------------ Announcing the Oxford Dictionaries API! The API offers world-renowned dictionary content that is easy and intuitive to access. Sign up for an account today to start using our lexical data to power your apps and projects. Get started today and enter our developer competition. http://sdm.link/oxford _______________________________________________ genode-main mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/genode-main
