x86_64: performance fixes when switching I/O port permissions

src/platform/x86_64/include/platform/context.h

@@ -44,6 +44,7 @@ typedef struct {
 
     ThreadGPR regs;         // register state
 
+    int iopl;               // set to 1 if I/O port privileges have been modified
     uint8_t ioports[8192];  // I/O port privileges
 } __attribute__((packed)) ThreadContext;
 

src/platform/x86_64/io.c

@@ -22,6 +22,7 @@ int platformIoperm(Thread *t, uintptr_t from, uintptr_t count, int enable) {
     if((from+count-1) > 0xFFFF) return -EINVAL;     // 65536 I/O ports on x86
 
     ThreadContext *ctx = (ThreadContext *) t->context;
+    ctx->iopl = 1;
 
     for(int i = 0; i < count; i++) {
         int byte = (from + i) / 8;

src/platform/x86_64/sched/context.c

@@ -97,8 +97,14 @@ void platformSwitchContext(Thread *t) {
     ThreadContext *ctx = (ThreadContext *)t->context;
     ctx->regs.rflags |= 0x202; // interrupts can never be switched off outside of the kernel
 
-    // modify the TSS with the current thread's I/O permissions
-    memcpy(kinfo->tss->ioports, ctx->ioports, 8192);
+    // modify the TSS with the current thread's I/O permissions if necessary
+    Thread *old = kinfo->thread;
+    ThreadContext *oldctx = NULL;
+    if(old) oldctx = (ThreadContext *) old->context;
+
+    if(ctx->iopl || (oldctx && oldctx->iopl)) {
+        memcpy(kinfo->tss->ioports, ctx->ioports, 8192);
+    }
 
     kinfo->thread = t;
     kinfo->process = getProcess(t->pid);