Kernel: Use the GS segment for the per-CPU struct

Right now we're using the FS segment for our per-CPU struct. On x86_64
there's an instruction to switch between a kernel and usermode GS
segment (swapgs) which we could use.

This patch doesn't update the rest of the code to use swapgs but it
prepares for that by using the GS segment instead of the FS segment.

Kernel/Arch/x86/ASM_wrapper.h

@@ -62,25 +62,25 @@ ALWAYS_INLINE u16 get_gs()
 }
 #endif
 
-ALWAYS_INLINE u32 read_fs_u32(u32 offset)
+ALWAYS_INLINE u32 read_gs_u32(u32 offset)
 {
     u32 val;
     asm volatile(
-        "movl %%fs:%a[off], %k[val]"
+        "movl %%gs:%a[off], %k[val]"
         : [val] "=r"(val)
         : [off] "ir"(offset));
     return val;
 }
 
-ALWAYS_INLINE FlatPtr read_fs_ptr(u32 offset)
+ALWAYS_INLINE FlatPtr read_gs_ptr(u32 offset)
 {
-    return read_fs_u32(offset);
+    return read_gs_u32(offset);
 }
 
-ALWAYS_INLINE void write_fs_u32(u32 offset, u32 val)
+ALWAYS_INLINE void write_gs_u32(u32 offset, u32 val)
 {
     asm volatile(
-        "movl %k[val], %%fs:%a[off]" ::[off] "ir"(offset), [val] "ir"(val)
+        "movl %k[val], %%gs:%a[off]" ::[off] "ir"(offset), [val] "ir"(val)
         : "memory");
 }
 

Kernel/Arch/x86/Processor.h

@@ -21,7 +21,7 @@ namespace Kernel {
 
 #if ARCH(X86_64)
 #    define MSR_FS_BASE 0xc0000100
-#    define MSR_GS_BASE 0xc0000102
+#    define MSR_GS_BASE 0xc0000101
 #endif
 
 class Thread;
@@ -241,16 +241,16 @@ public:
 
     ALWAYS_INLINE static Processor& current()
     {
-        return *(Processor*)read_fs_ptr(__builtin_offsetof(Processor, m_self));
+        return *(Processor*)read_gs_ptr(__builtin_offsetof(Processor, m_self));
     }
 
     ALWAYS_INLINE static bool is_initialized()
     {
         return
 #if ARCH(I386)
-            get_fs() == GDT_SELECTOR_PROC &&
+            get_gs() == GDT_SELECTOR_PROC &&
 #endif
-            read_fs_u32(__builtin_offsetof(Processor, m_self)) != 0;
+            read_gs_u32(__builtin_offsetof(Processor, m_self)) != 0;
     }
 
     ALWAYS_INLINE void set_scheduler_data(SchedulerPerProcessorData& scheduler_data)
@@ -286,19 +286,19 @@ public:
         // to another processor, which would lead us to get the wrong thread.
         // To avoid having to disable interrupts, we can just read the field
         // directly in an atomic fashion, similar to Processor::current.
-        return (Thread*)read_fs_ptr(__builtin_offsetof(Processor, m_current_thread));
+        return (Thread*)read_gs_ptr(__builtin_offsetof(Processor, m_current_thread));
     }
 
     ALWAYS_INLINE static void set_current_thread(Thread& current_thread)
     {
         // See comment in Processor::current_thread
-        write_fs_u32(__builtin_offsetof(Processor, m_current_thread), FlatPtr(&current_thread));
+        write_gs_u32(__builtin_offsetof(Processor, m_current_thread), FlatPtr(&current_thread));
     }
 
     ALWAYS_INLINE static Thread* idle_thread()
     {
         // See comment in Processor::current_thread
-        return (Thread*)read_fs_ptr(__builtin_offsetof(Processor, m_idle_thread));
+        return (Thread*)read_gs_ptr(__builtin_offsetof(Processor, m_idle_thread));
     }
 
     ALWAYS_INLINE u32 get_id() const
@@ -314,7 +314,7 @@ public:
     ALWAYS_INLINE static u32 id()
     {
         // See comment in Processor::current_thread
-        return read_fs_ptr(__builtin_offsetof(Processor, m_cpu));
+        return read_gs_ptr(__builtin_offsetof(Processor, m_cpu));
     }
 
     ALWAYS_INLINE static bool is_bootstrap_processor()

Kernel/Arch/x86/common/Interrupts.cpp

@@ -58,7 +58,7 @@ static EntropySource s_entropy_source_interrupts { EntropySource::Static::Interr
         "    mov %ax, %ds\n"                        \
         "    mov %ax, %es\n"                        \
         "    mov $" __STRINGIFY(GDT_SELECTOR_PROC) ", %ax\n" \
-        "    mov %ax, %fs\n"                        \
+        "    mov %ax, %gs\n"                        \
         "    pushl %esp \n" /* set TrapFrame::regs */ \
         "    subl $" __STRINGIFY(TRAP_FRAME_SIZE - 4) ", %esp \n" \
         "    pushl %esp \n"                         \
@@ -84,7 +84,7 @@ static EntropySource s_entropy_source_interrupts { EntropySource::Static::Interr
         "    mov %ax, %ds\n"                        \
         "    mov %ax, %es\n"                        \
         "    mov $" __STRINGIFY(GDT_SELECTOR_PROC) ", %ax\n" \
-        "    mov %ax, %fs\n"                        \
+        "    mov %ax, %gs\n"                        \
         "    pushl %esp \n" /* set TrapFrame::regs */ \
         "    subl $" __STRINGIFY(TRAP_FRAME_SIZE - 4) ", %esp \n" \
         "    pushl %esp \n"                         \

Kernel/Arch/x86/common/Processor.cpp

@@ -1092,17 +1092,17 @@ UNMAP_AFTER_INIT void Processor::gdt_init()
     tls_descriptor.type = 2;
     write_gdt_entry(GDT_SELECTOR_TLS, tls_descriptor); // tls3
 
-    Descriptor fs_descriptor {};
+    Descriptor gs_descriptor {};
-    fs_descriptor.set_base(VirtualAddress { this });
+    gs_descriptor.set_base(VirtualAddress { this });
-    fs_descriptor.set_limit(sizeof(Processor) - 1);
+    gs_descriptor.set_limit(sizeof(Processor) - 1);
-    fs_descriptor.dpl = 0;
+    gs_descriptor.dpl = 0;
-    fs_descriptor.segment_present = 1;
+    gs_descriptor.segment_present = 1;
-    fs_descriptor.granularity = 0;
+    gs_descriptor.granularity = 0;
-    fs_descriptor.operation_size64 = 0;
+    gs_descriptor.operation_size64 = 0;
-    fs_descriptor.operation_size32 = 1;
+    gs_descriptor.operation_size32 = 1;
-    fs_descriptor.descriptor_type = 1;
+    gs_descriptor.descriptor_type = 1;
-    fs_descriptor.type = 2;
+    gs_descriptor.type = 2;
-    write_gdt_entry(GDT_SELECTOR_PROC, fs_descriptor); // fs0
+    write_gdt_entry(GDT_SELECTOR_PROC, gs_descriptor); // gs0
 #endif
 
     Descriptor tss_descriptor {};
@@ -1127,16 +1127,16 @@ UNMAP_AFTER_INIT void Processor::gdt_init()
     load_task_register(GDT_SELECTOR_TSS);
 
 #if ARCH(X86_64)
-    MSR fs_base(MSR_FS_BASE);
+    MSR gs_base(MSR_GS_BASE);
-    fs_base.set((size_t)this & 0xffffffff, (size_t)this >> 32);
+    gs_base.set((size_t)this & 0xffffffff, (size_t)this >> 32);
 #else
     asm volatile(
         "mov %%ax, %%ds\n"
         "mov %%ax, %%es\n"
-        "mov %%ax, %%gs\n"
+        "mov %%ax, %%fs\n"
         "mov %%ax, %%ss\n" ::"a"(GDT_SELECTOR_DATA0)
         : "memory");
-    set_fs(GDT_SELECTOR_PROC);
+    set_gs(GDT_SELECTOR_PROC);
 #endif
 
 #if ARCH(I386)

Kernel/Arch/x86/i386/InterruptEntry.cpp

@@ -21,7 +21,7 @@ asm(
     "    mov %ax, %ds\n"
     "    mov %ax, %es\n"
     "    mov $" __STRINGIFY(GDT_SELECTOR_PROC) ", %ax\n"
-    "    mov %ax, %fs\n"
+    "    mov %ax, %gs\n"
     "    pushl %esp \n" // set TrapFrame::regs
     "    subl $" __STRINGIFY(TRAP_FRAME_SIZE - 4) ", %esp \n"
     "    movl %esp, %ebx \n" // save pointer to TrapFrame

Kernel/Arch/x86/i386/Processor.cpp

@@ -174,7 +174,7 @@ FlatPtr Processor::init_context(Thread& thread, bool leave_crit)
     regs.es = GDT_SELECTOR_DATA0;
     regs.gs = GDT_SELECTOR_DATA0;
     regs.ss = GDT_SELECTOR_DATA0;
-    regs.fs = GDT_SELECTOR_PROC;
+    regs.gs = GDT_SELECTOR_PROC;
     return stack_top;
 }
 

Kernel/Syscall.cpp

@@ -38,7 +38,7 @@ NEVER_INLINE void syscall_asm_entry_dummy()
         "    mov %ax, %ds\n"
         "    mov %ax, %es\n"
         "    mov $" __STRINGIFY(GDT_SELECTOR_PROC) ", %ax\n"
-        "    mov %ax, %fs\n"
+        "    mov %ax, %gs\n"
         "    cld\n"
         "    xor %esi, %esi\n"
         "    xor %edi, %edi\n"

Kernel/Thread.cpp

@@ -102,9 +102,9 @@ Thread::Thread(NonnullRefPtr<Process> process, NonnullOwnPtr<Region> kernel_stac
         m_regs.cs = GDT_SELECTOR_CODE0;
         m_regs.ds = GDT_SELECTOR_DATA0;
         m_regs.es = GDT_SELECTOR_DATA0;
-        m_regs.fs = GDT_SELECTOR_PROC;
+        m_regs.fs = 0;
         m_regs.ss = GDT_SELECTOR_DATA0;
-        m_regs.gs = 0;
+        m_regs.gs = GDT_SELECTOR_PROC;
     } else {
         m_regs.cs = GDT_SELECTOR_CODE3 | 3;
         m_regs.ds = GDT_SELECTOR_DATA3 | 3;