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diff --git a/Documentation/arch/ia64/fsys.rst b/Documentation/arch/ia64/fsys.rst deleted file mode 100644 index a702d2cc94..0000000000 --- a/Documentation/arch/ia64/fsys.rst +++ /dev/null @@ -1,303 +0,0 @@ -=================================== -Light-weight System Calls for IA-64 -=================================== - - Started: 13-Jan-2003 - - Last update: 27-Sep-2003 - - David Mosberger-Tang - <davidm@hpl.hp.com> - -Using the "epc" instruction effectively introduces a new mode of -execution to the ia64 linux kernel. We call this mode the -"fsys-mode". To recap, the normal states of execution are: - - - kernel mode: - Both the register stack and the memory stack have been - switched over to kernel memory. The user-level state is saved - in a pt-regs structure at the top of the kernel memory stack. - - - user mode: - Both the register stack and the kernel stack are in - user memory. The user-level state is contained in the - CPU registers. - - - bank 0 interruption-handling mode: - This is the non-interruptible state which all - interruption-handlers start execution in. The user-level - state remains in the CPU registers and some kernel state may - be stored in bank 0 of registers r16-r31. - -In contrast, fsys-mode has the following special properties: - - - execution is at privilege level 0 (most-privileged) - - - CPU registers may contain a mixture of user-level and kernel-level - state (it is the responsibility of the kernel to ensure that no - security-sensitive kernel-level state is leaked back to - user-level) - - - execution is interruptible and preemptible (an fsys-mode handler - can disable interrupts and avoid all other interruption-sources - to avoid preemption) - - - neither the memory-stack nor the register-stack can be trusted while - in fsys-mode (they point to the user-level stacks, which may - be invalid, or completely bogus addresses) - -In summary, fsys-mode is much more similar to running in user-mode -than it is to running in kernel-mode. Of course, given that the -privilege level is at level 0, this means that fsys-mode requires some -care (see below). - - -How to tell fsys-mode -===================== - -Linux operates in fsys-mode when (a) the privilege level is 0 (most -privileged) and (b) the stacks have NOT been switched to kernel memory -yet. For convenience, the header file <asm-ia64/ptrace.h> provides -three macros:: - - user_mode(regs) - user_stack(task,regs) - fsys_mode(task,regs) - -The "regs" argument is a pointer to a pt_regs structure. The "task" -argument is a pointer to the task structure to which the "regs" -pointer belongs to. user_mode() returns TRUE if the CPU state pointed -to by "regs" was executing in user mode (privilege level 3). -user_stack() returns TRUE if the state pointed to by "regs" was -executing on the user-level stack(s). Finally, fsys_mode() returns -TRUE if the CPU state pointed to by "regs" was executing in fsys-mode. -The fsys_mode() macro is equivalent to the expression:: - - !user_mode(regs) && user_stack(task,regs) - -How to write an fsyscall handler -================================ - -The file arch/ia64/kernel/fsys.S contains a table of fsyscall-handlers -(fsyscall_table). This table contains one entry for each system call. -By default, a system call is handled by fsys_fallback_syscall(). This -routine takes care of entering (full) kernel mode and calling the -normal Linux system call handler. For performance-critical system -calls, it is possible to write a hand-tuned fsyscall_handler. For -example, fsys.S contains fsys_getpid(), which is a hand-tuned version -of the getpid() system call. - -The entry and exit-state of an fsyscall handler is as follows: - -Machine state on entry to fsyscall handler ------------------------------------------- - - ========= =============================================================== - r10 0 - r11 saved ar.pfs (a user-level value) - r15 system call number - r16 "current" task pointer (in normal kernel-mode, this is in r13) - r32-r39 system call arguments - b6 return address (a user-level value) - ar.pfs previous frame-state (a user-level value) - PSR.be cleared to zero (i.e., little-endian byte order is in effect) - - all other registers may contain values passed in from user-mode - ========= =============================================================== - -Required machine state on exit to fsyscall handler --------------------------------------------------- - - ========= =========================================================== - r11 saved ar.pfs (as passed into the fsyscall handler) - r15 system call number (as passed into the fsyscall handler) - r32-r39 system call arguments (as passed into the fsyscall handler) - b6 return address (as passed into the fsyscall handler) - ar.pfs previous frame-state (as passed into the fsyscall handler) - ========= =========================================================== - -Fsyscall handlers can execute with very little overhead, but with that -speed comes a set of restrictions: - - * Fsyscall-handlers MUST check for any pending work in the flags - member of the thread-info structure and if any of the - TIF_ALLWORK_MASK flags are set, the handler needs to fall back on - doing a full system call (by calling fsys_fallback_syscall). - - * Fsyscall-handlers MUST preserve incoming arguments (r32-r39, r11, - r15, b6, and ar.pfs) because they will be needed in case of a - system call restart. Of course, all "preserved" registers also - must be preserved, in accordance to the normal calling conventions. - - * Fsyscall-handlers MUST check argument registers for containing a - NaT value before using them in any way that could trigger a - NaT-consumption fault. If a system call argument is found to - contain a NaT value, an fsyscall-handler may return immediately - with r8=EINVAL, r10=-1. - - * Fsyscall-handlers MUST NOT use the "alloc" instruction or perform - any other operation that would trigger mandatory RSE - (register-stack engine) traffic. - - * Fsyscall-handlers MUST NOT write to any stacked registers because - it is not safe to assume that user-level called a handler with the - proper number of arguments. - - * Fsyscall-handlers need to be careful when accessing per-CPU variables: - unless proper safe-guards are taken (e.g., interruptions are avoided), - execution may be pre-empted and resumed on another CPU at any given - time. - - * Fsyscall-handlers must be careful not to leak sensitive kernel' - information back to user-level. In particular, before returning to - user-level, care needs to be taken to clear any scratch registers - that could contain sensitive information (note that the current - task pointer is not considered sensitive: it's already exposed - through ar.k6). - - * Fsyscall-handlers MUST NOT access user-memory without first - validating access-permission (this can be done typically via - probe.r.fault and/or probe.w.fault) and without guarding against - memory access exceptions (this can be done with the EX() macros - defined by asmmacro.h). - -The above restrictions may seem draconian, but remember that it's -possible to trade off some of the restrictions by paying a slightly -higher overhead. For example, if an fsyscall-handler could benefit -from the shadow register bank, it could temporarily disable PSR.i and -PSR.ic, switch to bank 0 (bsw.0) and then use the shadow registers as -needed. In other words, following the above rules yields extremely -fast system call execution (while fully preserving system call -semantics), but there is also a lot of flexibility in handling more -complicated cases. - -Signal handling -=============== - -The delivery of (asynchronous) signals must be delayed until fsys-mode -is exited. This is accomplished with the help of the lower-privilege -transfer trap: arch/ia64/kernel/process.c:do_notify_resume_user() -checks whether the interrupted task was in fsys-mode and, if so, sets -PSR.lp and returns immediately. When fsys-mode is exited via the -"br.ret" instruction that lowers the privilege level, a trap will -occur. The trap handler clears PSR.lp again and returns immediately. -The kernel exit path then checks for and delivers any pending signals. - -PSR Handling -============ - -The "epc" instruction doesn't change the contents of PSR at all. This -is in contrast to a regular interruption, which clears almost all -bits. Because of that, some care needs to be taken to ensure things -work as expected. The following discussion describes how each PSR bit -is handled. - -======= ======================================================================= -PSR.be Cleared when entering fsys-mode. A srlz.d instruction is used - to ensure the CPU is in little-endian mode before the first - load/store instruction is executed. PSR.be is normally NOT - restored upon return from an fsys-mode handler. In other - words, user-level code must not rely on PSR.be being preserved - across a system call. -PSR.up Unchanged. -PSR.ac Unchanged. -PSR.mfl Unchanged. Note: fsys-mode handlers must not write-registers! -PSR.mfh Unchanged. Note: fsys-mode handlers must not write-registers! -PSR.ic Unchanged. Note: fsys-mode handlers can clear the bit, if needed. -PSR.i Unchanged. Note: fsys-mode handlers can clear the bit, if needed. -PSR.pk Unchanged. -PSR.dt Unchanged. -PSR.dfl Unchanged. Note: fsys-mode handlers must not write-registers! -PSR.dfh Unchanged. Note: fsys-mode handlers must not write-registers! -PSR.sp Unchanged. -PSR.pp Unchanged. -PSR.di Unchanged. -PSR.si Unchanged. -PSR.db Unchanged. The kernel prevents user-level from setting a hardware - breakpoint that triggers at any privilege level other than - 3 (user-mode). -PSR.lp Unchanged. -PSR.tb Lazy redirect. If a taken-branch trap occurs while in - fsys-mode, the trap-handler modifies the saved machine state - such that execution resumes in the gate page at - syscall_via_break(), with privilege level 3. Note: the - taken branch would occur on the branch invoking the - fsyscall-handler, at which point, by definition, a syscall - restart is still safe. If the system call number is invalid, - the fsys-mode handler will return directly to user-level. This - return will trigger a taken-branch trap, but since the trap is - taken _after_ restoring the privilege level, the CPU has already - left fsys-mode, so no special treatment is needed. -PSR.rt Unchanged. -PSR.cpl Cleared to 0. -PSR.is Unchanged (guaranteed to be 0 on entry to the gate page). -PSR.mc Unchanged. -PSR.it Unchanged (guaranteed to be 1). -PSR.id Unchanged. Note: the ia64 linux kernel never sets this bit. -PSR.da Unchanged. Note: the ia64 linux kernel never sets this bit. -PSR.dd Unchanged. Note: the ia64 linux kernel never sets this bit. -PSR.ss Lazy redirect. If set, "epc" will cause a Single Step Trap to - be taken. The trap handler then modifies the saved machine - state such that execution resumes in the gate page at - syscall_via_break(), with privilege level 3. -PSR.ri Unchanged. -PSR.ed Unchanged. Note: This bit could only have an effect if an fsys-mode - handler performed a speculative load that gets NaTted. If so, this - would be the normal & expected behavior, so no special treatment is - needed. -PSR.bn Unchanged. Note: fsys-mode handlers may clear the bit, if needed. - Doing so requires clearing PSR.i and PSR.ic as well. -PSR.ia Unchanged. Note: the ia64 linux kernel never sets this bit. -======= ======================================================================= - -Using fast system calls -======================= - -To use fast system calls, userspace applications need simply call -__kernel_syscall_via_epc(). For example - --- example fgettimeofday() call -- - --- fgettimeofday.S -- - -:: - - #include <asm/asmmacro.h> - - GLOBAL_ENTRY(fgettimeofday) - .prologue - .save ar.pfs, r11 - mov r11 = ar.pfs - .body - - mov r2 = 0xa000000000020660;; // gate address - // found by inspection of System.map for the - // __kernel_syscall_via_epc() function. See - // below for how to do this for real. - - mov b7 = r2 - mov r15 = 1087 // gettimeofday syscall - ;; - br.call.sptk.many b6 = b7 - ;; - - .restore sp - - mov ar.pfs = r11 - br.ret.sptk.many rp;; // return to caller - END(fgettimeofday) - --- end fgettimeofday.S -- - -In reality, getting the gate address is accomplished by two extra -values passed via the ELF auxiliary vector (include/asm-ia64/elf.h) - - * AT_SYSINFO : is the address of __kernel_syscall_via_epc() - * AT_SYSINFO_EHDR : is the address of the kernel gate ELF DSO - -The ELF DSO is a pre-linked library that is mapped in by the kernel at -the gate page. It is a proper ELF shared object so, with a dynamic -loader that recognises the library, you should be able to make calls to -the exported functions within it as with any other shared library. -AT_SYSINFO points into the kernel DSO at the -__kernel_syscall_via_epc() function for historical reasons (it was -used before the kernel DSO) and as a convenience. |