From a7ca32e3a3ec2d3c1947a06fbcde0f779b0b1fec Mon Sep 17 00:00:00 2001
From: Frans Kaashoek <kaashoek@mit.edu>
Date: Tue, 9 Oct 2018 20:22:48 -0400
Subject: [PATCH] Change mycpu() to use %gs.

---
 defs.h |  1 +
 main.c |  2 ++
 proc.c | 16 ++++++++++++----
 proc.h |  1 +
 vm.c   |  7 +++++--
 5 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/defs.h b/defs.h
index 763eb75..3735f6f 100644
--- a/defs.h
+++ b/defs.h
@@ -110,6 +110,7 @@ int             fork(void);
 int             growproc(int);
 int             kill(int);
 struct cpu*     mycpu(void);
+struct cpu*     getmycpu(void);
 struct proc*    myproc();
 void            pinit(void);
 void            procdump(void);
diff --git a/main.c b/main.c
index a900e9d..3127b15 100644
--- a/main.c
+++ b/main.c
@@ -45,6 +45,8 @@ main(uint64 mbmagic, uint64 mbaddr)
   return 0;
 }
 
+extern struct cpu* getmycpu();
+
 // Common CPU setup code.
 static void
 mpmain(void)
diff --git a/proc.c b/proc.c
index 55e435c..3c0acbd 100644
--- a/proc.c
+++ b/proc.c
@@ -37,16 +37,15 @@ cpuid() {
 // Must be called with interrupts disabled to avoid the caller being
 // rescheduled between reading lapicid and running through the loop.
 struct cpu*
-mycpu(void)
+getmycpu(void)
 {
   int apicid, i;
   
   if(readeflags()&FL_IF)
-    panic("mycpu called with interrupts enabled\n");
+    panic("getmycpu called with interrupts enabled\n");
   
   apicid = lapicid();
-  // APIC IDs are not guaranteed to be contiguous. Maybe we should have
-  // a reverse map, or reserve a register to store &cpus[i].
+  // APIC IDs are not guaranteed to be contiguous.
   for (i = 0; i < ncpu; ++i) {
     if (cpus[i].apicid == apicid)
       return &cpus[i];
@@ -54,6 +53,15 @@ mycpu(void)
   panic("unknown apicid\n");
 }
 
+// Return this core's cpu struct using %gs. %gs points this core's struct
+// cpu. Offet 24 in struct cpu is cpu.
+struct cpu*
+mycpu(void) {
+  struct cpu *c;
+  asm volatile("mov %%gs:24, %0" : "=r" (c));
+  return c;
+}
+
 // Disable interrupts so that we are not rescheduled
 // while reading proc from the cpu structure
 struct proc*
diff --git a/proc.h b/proc.h
index 9261766..0b01b75 100644
--- a/proc.h
+++ b/proc.h
@@ -3,6 +3,7 @@ struct cpu {
   uint64 syscallno;            // Temporary used by sysentry
   uint64 usp;                  // Temporary used by sysentry
   struct proc *proc;           // The process running on this cpu or null
+  struct cpu *cpu;             // XXX
   uchar apicid;                // Local APIC ID
   struct context *scheduler;   // swtch() here to enter scheduler
   struct taskstate ts;         // Used by x86 to find stack for interrupt
diff --git a/vm.c b/vm.c
index ebb6749..c0276a1 100644
--- a/vm.c
+++ b/vm.c
@@ -39,7 +39,8 @@ seginit(void)
   struct cpu *c;
   struct desctr dtr;
 
-  c = mycpu();
+  c = getmycpu();
+  
   memmove(c->gdt, bootgdt, sizeof bootgdt);
   dtr.limit = sizeof(c->gdt)-1;
   dtr.base = (uint64) c->gdt;
@@ -54,10 +55,12 @@ seginit(void)
   writemsr(MSR_LSTAR, (uint64)&sysentry);
   writemsr(MSR_SFMASK, FL_TF | FL_IF);
 
-    // Initialize cpu-local storage.
+  // Initialize cpu-local storage so that each core can easily
+  // find its struct cpu using %gs.
   writegs(SEG_KDATA);
   writemsr(MSR_GS_BASE, (uint64)c);
   writemsr(MSR_GS_KERNBASE, (uint64)c);
+  c->cpu = c;
 }
 
 // Return the address of the PTE in page table pgdir