How are the arguments and system call number passed while executing SVC instruction (for enter kernel mode from user mode in Linux)?
Ans. : Simply entering kernel-space with the help of a system call alone is not sufficient because there are multiple system calls, all of which enter the kernel in the same manner. Thus, the system call number must be passed into the kernel. On x86, the syscall number is fed to the kernel via the eax register. Before causing the trap into the kernel, user-space sticks in eax the number corresponding to the desired system call. The system call handler then reads the value from eax. Other architectures do something similar.
The system_call() function checks the validity of the given system call number by comparing it to NR_syscalls. If it is larger than or equal to NR_syscalls, the function returns -ENOSYS. Otherwise, the specified system call is invoked:
call *sys_call_table(,%eax,4)
In addition to the system call number, most syscalls require that one or more parameters be passed to them. Somehow, user-space must relay the parameters to the kernel during the trap. The easiest way to do this is via the same means that the syscall number is passed: The parameters are stored in registers. On x86, the registers ebx, ecx, edx, esi, and edi contain, in order, the first five arguments. In the unlikely case of six or more arguments, a single register is used to hold a pointer to user-space where all the parameters are stored.
The return value is sent to user-space also via register. On x86, it is written into the eax register.
Ans.: Independent memory operations are effectively performed in random order, but this can be a problem for CPU-CPU interaction and for I/O. What is required is some way of intervening to instruct the compiler and the CPU to restrict the order.
Memory barriers are such interventions. They impose a perceived partial ordering over the memory operations on either side of the barrier.
Such enforcement is important because the CPUs and other devices in a system can use a variety of tricks to improve performance, including reordering, deferral and combination of memory operations; speculative loads; speculative branch prediction and various types of caching. Memory barriers are used to override or suppress these tricks, allowing the code to sanely control the interaction of multiple CPUs and/or devices.
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I have installed in my laptop Linux operating system Ubuntu. I still have Windows 7 on computer, I only choose which operating system I want to use when I turn on computer. But the problem is that when I'm using Ubuntu I have only 17 GB space on HD(full is 250 GB). I also cannot access other files such as music or films that are on HD (outside of those 17 GB).
But nice Article Mate! Great Information! Keep up the good work!
How are the arguments and system call number passed while executing SVC instruction (for enter kernel mode from user mode in Linux)?
ReplyDeleteAns. :
Simply entering kernel-space with the help of a system call alone is not sufficient because there are multiple system calls, all of which enter the kernel in the same manner. Thus, the system call number must be passed into the kernel. On x86, the syscall number is fed to the kernel via the eax register. Before causing the trap into the kernel, user-space sticks in eax the number corresponding to the desired system call. The system call handler then reads the value from eax. Other architectures do something similar.
The system_call() function checks the validity of the given system call number by comparing it to NR_syscalls. If it is larger than or equal to NR_syscalls, the function returns -ENOSYS. Otherwise, the specified system call is invoked:
call *sys_call_table(,%eax,4)
In addition to the system call number, most syscalls require that one or more parameters be passed to them. Somehow, user-space must relay the parameters to the kernel during the trap. The easiest way to do this is via the same means that the syscall number is passed: The parameters are stored in registers. On x86, the registers ebx, ecx, edx, esi, and edi contain, in order, the first five arguments. In the unlikely case of six or more arguments, a single register is used to hold a pointer to user-space where all the parameters are stored.
The return value is sent to user-space also via register. On x86, it is written into the eax register.
Refer: http://www.makelinux.com/books/lkd2/ch05lev1sec3
DeleteWhat are memory barriers?
ReplyDeleteAns.:
Independent memory operations are effectively performed
in random order, but this can be a problem for CPU-CPU interaction and for I/O.
What is required is some way of intervening to instruct the compiler and the
CPU to restrict the order.
Memory barriers are such interventions. They impose a perceived partial
ordering over the memory operations on either side of the barrier.
Such enforcement is important because the CPUs and other devices in a system
can use a variety of tricks to improve performance, including reordering,
deferral and combination of memory operations; speculative loads; speculative
branch prediction and various types of caching. Memory barriers are used to
override or suppress these tricks, allowing the code to sanely control the
interaction of multiple CPUs and/or devices.
Refer: https://www.kernel.org/doc/Documentation/memory-barriers.txt
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ReplyDeleteThree cheers to you ! Hooray!!! I feel like I hit the jackpot on Embedded systems and linux device driver interview questions !
I have installed in my laptop Linux operating system Ubuntu. I still have Windows 7 on computer, I only choose which operating system I want to use when I turn on computer. But the problem is that when I'm using Ubuntu I have only 17 GB space on HD(full is 250 GB). I also cannot access other files such as music or films that are on HD (outside of those 17 GB).
But nice Article Mate! Great Information! Keep up the good work!
Ciao,
Radhey