Kernel: Pass current cpu index as a GDT limit

I had no idea LSL was an instruction. This cleans up code to get the
current cpu by a lot and does not require extra segment usage :D

kernel/include/kernel/API/SharedPage.h

@@ -12,7 +12,7 @@ namespace Kernel::API
 
 	struct SharedPage
 	{
-		uint8_t __sequence[0x100];
+		uint16_t gdt_cpu_offset;
 
 		uint32_t features;
 

kernel/include/kernel/GDT.h

@@ -133,6 +133,12 @@ namespace Kernel
 		void set_gsbase(uintptr_t addr);
 #endif
 
+		static uint16_t cpu_index_offset() { return m_cpu_index_offset; }
+		void set_cpu_index(uint8_t index)
+		{
+			write_entry(m_cpu_index_offset, 0, index, 0xF2, 0x4);
+		}
+
 	private:
 		GDT() = default;
 
@@ -151,11 +157,13 @@ namespace Kernel
 
 	private:
 #if ARCH(x86_64)
-		BAN::Array<SegmentDescriptor, 8> m_gdt; // null, kernel code, kernel data, user code (32 bit), user data, user code (64 bit), tss low, tss high
+		BAN::Array<SegmentDescriptor, 9> m_gdt; // null, kernel code, kernel data, user code (32 bit), user data, user code (64 bit), cpu-index, tss low, tss high
-		static constexpr uint16_t m_tss_offset = 0x30;
+		static constexpr uint16_t m_cpu_index_offset = 0x30;
+		static constexpr uint16_t m_tss_offset = 0x38;
 #elif ARCH(i686)
-		BAN::Array<SegmentDescriptor, 9> m_gdt; // null, kernel code, kernel data, user code, user data, processor data, fsbase, gsbase, tss
+		BAN::Array<SegmentDescriptor, 10> m_gdt; // null, kernel code, kernel data, user code, user data, processor data, fsbase, gsbase, cpu-index, tss
-		static constexpr uint16_t m_tss_offset = 0x40;
+		static constexpr uint16_t m_cpu_index_offset = 0x40;
+		static constexpr uint16_t m_tss_offset = 0x48;
 #endif
 		TaskStateSegment m_tss;
 		const GDTR m_gdtr {

kernel/include/kernel/Thread.h

@@ -178,9 +178,7 @@ namespace Kernel
 		bool                       m_is_userspace         { false };
 		bool                       m_delete_process       { false };
 
-		bool                       m_has_custom_fsbase    { false };
 		vaddr_t                    m_fsbase               { 0 };
-		bool                       m_has_custom_gsbase    { false };
 		vaddr_t                    m_gsbase               { 0 };
 
 		SchedulerQueue::Node*      m_scheduler_node       { nullptr };

kernel/kernel/GDT.cpp

@@ -32,6 +32,8 @@ namespace Kernel
 		gdt->write_entry(0x38, 0x00000000, 0x00000, 0xF2, 0xC); // gsbase
 #endif
 
+		gdt->write_entry(m_cpu_index_offset, 0, 0, 0, 0);
+
 		gdt->write_tss();
 
 		return gdt;

kernel/kernel/Process.cpp

@@ -3314,7 +3314,6 @@ namespace Kernel
 	BAN::ErrorOr<long> Process::sys_set_fsbase(void* addr)
 	{
 		auto& thread = Thread::current();
-		thread.m_has_custom_fsbase = true;
 		thread.set_fsbase(reinterpret_cast<vaddr_t>(addr));
 		Processor::load_fsbase();
 		return 0;
@@ -3328,7 +3327,6 @@ namespace Kernel
 	BAN::ErrorOr<long> Process::sys_set_gsbase(void* addr)
 	{
 		auto& thread = Thread::current();
-		thread.m_has_custom_gsbase = true;
 		thread.set_gsbase(reinterpret_cast<vaddr_t>(addr));
 		Processor::load_gsbase();
 		return 0;

kernel/kernel/Processor.cpp

@@ -217,8 +217,7 @@ namespace Kernel
 		memset(reinterpret_cast<void*>(s_shared_page_vaddr), 0, PAGE_SIZE);
 
 		auto& shared_page = *reinterpret_cast<volatile API::SharedPage*>(s_shared_page_vaddr);
-		for (size_t i = 0; i <= 0xFF; i++)
+		shared_page.gdt_cpu_offset = GDT::cpu_index_offset();
-			shared_page.__sequence[i] = i;
 		shared_page.features = 0;
 
 		ASSERT(Processor::count() + sizeof(Kernel::API::SharedPage) <= PAGE_SIZE);

kernel/kernel/Thread.cpp

@@ -303,22 +303,7 @@ namespace Kernel
 	{
 		if (!is_userspace() || !has_process())
 			return;
-
+		Processor::gdt().set_cpu_index(Processor::current_index());
-#if ARCH(x86_64)
-		if (m_has_custom_gsbase)
-			return;
-#elif ARCH(i686)
-		if (m_has_custom_fsbase)
-			return;
-#endif
-
-		const vaddr_t vaddr = process().shared_page_vaddr() + Processor::current_index();
-
-#if ARCH(x86_64)
-		set_gsbase(vaddr);
-#elif ARCH(i686)
-		set_fsbase(vaddr);
-#endif
 	}
 
 	BAN::ErrorOr<Thread*> Thread::pthread_create(entry_t entry, void* arg)

userspace/libraries/LibC/sched.cpp

@@ -27,12 +27,7 @@ int sched_getcpu(void)
 {
 	if (g_shared_page == nullptr)
 		return -1;
-
+	uint16_t limit;
-	uint8_t cpu;
+	asm volatile("lsl %1, %0" : "=r"(limit) : "r"(g_shared_page->gdt_cpu_offset));
-#if defined(__x86_64__)
+	return limit;
-	asm volatile("movb %%gs:0, %0" : "=r"(cpu));
-#elif defined(__i686__)
-	asm volatile("movb %%fs:0, %0" : "=q"(cpu));
-#endif
-	return cpu;
 }

userspace/libraries/LibC/time.cpp

@@ -30,13 +30,9 @@ int clock_gettime(clockid_t clock_id, struct timespec* tp)
 
 	const auto get_cpu =
 		[]() -> uint8_t {
-			uint8_t cpu;
+			uint16_t limit;
-#if defined(__x86_64__)
+			asm volatile("lsl %1, %0" : "=r"(limit) : "r"(g_shared_page->gdt_cpu_offset));
-			asm volatile("movb %%gs:0, %0" : "=r"(cpu));
+			return limit;
-#elif defined(__i686__)
-			asm volatile("movb %%fs:0, %0" : "=q"(cpu));
-#endif
-			return cpu;
 		};
 
 	for (;;)