src/system/kernel/arch/x86/64/syscalls.cpp

   1 /*
   2  * Copyright 2012, Alex Smith, alex@alex-smith.me.uk.
   3  * Distributed under the terms of the MIT License.
   4  */
   5
   6
   7 #include "x86_syscalls.h"
   8
   9 #include <KernelExport.h>
  10
  11 #include <cpu.h>
  12 #include <smp.h>
  13
  14
  15 // SYSCALL handler (in interrupts.S).
  16 extern "C" void x86_64_syscall_entry(void);
  17
  18
  19 static void
  20 init_syscall_registers(void* dummy, int cpuNum)
  21 {
  22         // Enable SYSCALL (EFER.SCE = 1).
  23         x86_write_msr(IA32_MSR_EFER, x86_read_msr(IA32_MSR_EFER)
  24                 | IA32_MSR_EFER_SYSCALL);
  25
  26         // Flags to clear upon entry. Want interrupts disabled and the direction
  27         // flag cleared.
  28         x86_write_msr(IA32_MSR_FMASK, X86_EFLAGS_INTERRUPT | X86_EFLAGS_DIRECTION);
  29
  30         // Entry point address.
  31         x86_write_msr(IA32_MSR_LSTAR, (addr_t)x86_64_syscall_entry);
  32
  33         // Segments that will be set upon entry and return. This is very strange
  34         // and requires a specific ordering of segments in the GDT. Upon entry:
  35         //  - CS is set to IA32_STAR[47:32]
  36         //  - SS is set to IA32_STAR[47:32] + 8
  37         // Upon return:
  38         //  - CS is set to IA32_STAR[63:48] + 16
  39         //  - SS is set to IA32_STAR[63:48] + 8
  40         // From this we get:
  41         //  - Entry CS  = KERNEL_CODE_SELECTOR
  42         //  - Entry SS  = KERNEL_CODE_SELECTOR + 8  = KERNEL_DATA_SELECTOR
  43         //  - Return CS = KERNEL_DATA_SELECTOR + 16 = USER_CODE_SELECTOR
  44         //  - Return SS = KERNEL_DATA_SELECTOR + 8  = USER_DATA_SLECTORG
  45         x86_write_msr(IA32_MSR_STAR, ((uint64)(KERNEL_DATA_SELECTOR | 3) << 48)
  46                 | ((uint64)KERNEL_CODE_SELECTOR << 32));
  47 }
  48
  49
  50 // #pragma mark -
  51
  52
  53 void
  54 x86_initialize_syscall(void)
  55 {
  56         // SYSCALL/SYSRET are always available on x86_64 so we just use them, no
  57         // need to use the commpage. Tell all CPUs to initialize the SYSCALL MSRs.
  58         call_all_cpus_sync(&init_syscall_registers, NULL);
  59 }