xv6解析-- 多處理器操做

xv6能夠運行多cpu的計算機上，這個os使用mycpu函數來標識初當前的cpu，使用struct cpu結構體來記錄當前的CPU狀態。使用cpus這些狀態存放於cpus數組中，使用ncpu來標誌cpu的個數。

  1 // Per-CPU state
  2 struct cpu {
  3   uchar apicid;                // Local APIC ID 每一個cpu都有一個惟一硬件ID，這個ID能夠lapicid()函數進行獲取，而後存放於這個字段中。
  4   struct context *scheduler;   // swtch() here to enter scheduler
  5   struct taskstate ts;         // Used by x86 to find stack for interrupt
  6   struct segdesc gdt[NSEGS];   // x86 global descriptor table
  7   volatile uint started;       // Has the CPU started?
  8   int ncli;                    // Depth of pushcli nesting.
  9   int intena;                  // Were interrupts enabled before pushcli?
 10   struct proc *proc;           // The process running on this cpu or null
 11 };
 12
 13 extern struct cpu cpus[NCPU]; //當前系統中存在的CPU
 14 extern int ncpu;

 

如下函數主要功能是獲取 運行當前代碼的cpu對應的ID，而後經過這個ID在cpus數組中進行匹配，獲得當前CPU信息（這個cpu信息是已經事先經過程序獲取的）

 35 // Must be called with interrupts disabled to avoid the caller being
 36 // rescheduled between reading lapicid and running through the loop.
 37 struct cpu*
 38 mycpu(void)
 39 {
 40   int apicid, i;
 41
 42   if(readeflags()&FL_IF)
 43     panic("mycpu called with interrupts enabled\n");
 44
 45   apicid = lapicid();
 46   // APIC IDs are not guaranteed to be contiguous. Maybe we should have
 47   // a reverse map, or reserve a register to store &cpus[i].
 48   for (i = 0; i < ncpu; ++i) {
 49     if (cpus[i].apicid == apicid)
 50       return &cpus[i];
 51   }
 52   panic("unknown apicid\n");
 53 }