add basic gdb support for aarch64 (#94)

following kernel features are required:
- first page table (pl0) is located at a fix address
- unhandled page faults are forwarded to the hypervisor

arch/aarch64/include/asm/page.h

@@ -179,12 +179,15 @@ int page_unmap(size_t viraddr, size_t npages);
 int page_set_flags(size_t viraddr, uint32_t npages, int flags);
 
 /** @brief Handler to map on demand pages for the heap
+ *
+ * * @param viraddr Virtual address, which triggers the page fault
+ * * @param pc Instruction pointer, where the handler is triggered
  *
  * @return
  * - 0 on success
  * - -EINVAL (-22) on failure.
  */
-int page_fault_handler(size_t viraddr);
+int page_fault_handler(size_t viraddr, size_t pc);
 
 /** @brief Flush Translation Lookaside Buffer
  */

arch/aarch64/kernel/entry.S

@@ -127,6 +127,23 @@ host_logical_addr: .quad 0
 .global uart_mmio
 uart_mmio: .dword 0
 
+.global l0_pgtable
+.align 12
+l0_pgtable:
+    .quad l1_pgtable + PT_PT
+    .space 510*8, 0
+    .quad l0_pgtable + PT_PT + PT_SELF
+l1_pgtable:
+    .quad l2_pgtable + PT_PT
+    .space 511*8, 0
+l2_pgtable:
+    .quad l3_pgtable + PT_PT
+    .space 511*8, 0
+l3_pgtable:
+    .quad 0x00000000 + PT_MEM_CD // map II ports
+    .quad 0x09000000 + PT_MEM_CD // map QEMU's uart port
+    .space 510*8, 0
+
 start64:
   //mrs x0, s3_1_c15_c3_0 // Read EL1 Configuration Base Address Register
 
@@ -572,21 +589,3 @@ END(vector_table)
 boot_stack: .skip KERNEL_STACK_SIZE
 .global boot_ist
 boot_ist: .skip KERNEL_STACK_SIZE
-
-
-.global l0_pgtable
-.align 12
-l0_pgtable:
-    .quad l1_pgtable + PT_PT
-    .space 510*8, 0
-    .quad l0_pgtable + PT_PT + PT_SELF
-l1_pgtable:
-    .quad l2_pgtable + PT_PT
-    .space 511*8, 0
-l2_pgtable:
-    .quad l3_pgtable + PT_PT
-    .space 511*8, 0
-l3_pgtable:
-    .quad 0x00000000 + PT_MEM_CD // map II ports
-    .quad 0x09000000 + PT_MEM_CD // map QEMU's uart port
-    .space 510*8, 0

arch/aarch64/kernel/irq.c

@@ -274,6 +274,7 @@ void do_sync(void *regs)
 	uint32_t esr = read_esr();
 	uint32_t ec = esr >> 26;
 	uint32_t iss = esr & 0xFFFFFF;
+	uint64_t pc = get_elr();
 
         /* data abort from lower or current level */
 	if ((ec == 0b100100) || (ec == 0b100101)) {
@@ -282,7 +283,7 @@ void do_sync(void *regs)
 			/* read far_el1 register, which holds the faulting virtual address */
 			uint64_t far = read_far();
 
-			if (page_fault_handler(far) == 0)
+			if (page_fault_handler(far, pc) == 0)
 				return;
 
 			LOG_ERROR("Unable to handle page fault at 0x%llx\n", far);
@@ -298,13 +299,14 @@ void do_sync(void *regs)
 		} else {
 			LOG_ERROR("Unknown exception\n");
 		}
+	} else if (ec == 0x3c) {
+		LOG_ERROR("Trap to debugger, PC=0x%x\n", pc);
 	} else {
-		LOG_ERROR("Unsupported exception class\n");
+		LOG_ERROR("Unsupported exception class: 0x%x, PC=0x%x\n", ec, pc);
 	}
 
-	while (1) {
-		HALT;
-	}
+	sys_exit(-EFAULT);
+
 }
 
 size_t** do_fiq(void *regs)

arch/aarch64/mm/page.c

@@ -44,6 +44,13 @@
 
 #include <asm/irq.h>
 #include <asm/page.h>
+#include <asm/uhyve.h>
+
+typedef struct {
+	uint64_t rip;
+	uint64_t addr;
+	int success;
+} __attribute__ ((packed)) uhyve_pfault_t;
 
 /* Note that linker symbols are not variables, they have no memory
  * allocated for maintaining a value, rather their address is their value. */
@@ -192,7 +199,7 @@ int page_unmap(size_t viraddr, size_t npages)
 	return 0;
 }
 
-int page_fault_handler(size_t viraddr)
+int page_fault_handler(size_t viraddr, size_t pc)
 {
 	task_t* task = per_core(current_task);
 
@@ -258,6 +265,14 @@ int page_fault_handler(size_t viraddr)
 default_handler:
 	spinlock_irqsave_unlock(&page_lock);
 
+	/* indicate unrecoverable page fault to the hypervisor */
+	uhyve_pfault_t arg = {pc, viraddr, -1};
+	/*
+	 * In principle, uhyve_send is a function call to the hypervisor.
+	 * => It is safe to pass data, which are stored on the stack.
+	 */
+	uhyve_send(UHYVE_PORT_PFAULT, (unsigned)virt_to_phys((size_t)&arg));
+
 	return -EINVAL;
 }
 

include/hermit/stddef.h

@@ -66,6 +66,8 @@ extern size_t image_size;
 #define UHYVE_PORT_EXIT			0x540
 #define UHYVE_PORT_LSEEK		0x580
 
+#define UHYVE_PORT_PFAULT		0x511
+
 // Networkports
 #define UHYVE_PORT_NETINFO		0x600
 #define UHYVE_PORT_NETWRITE		0x640