Revert "Revert "Handle exceptions.""
This reverts commit 81334588b1ccc9ce36ee5b95d38950d5e4dbb8f2.
Reason for revert: Relanding original change with fix.
Bug: 223166344
Test: Ran unprotected VM under crosvm.
Change-Id: Ia1a73afef6df10a8fecec8321233b4d8855329be
diff --git a/pvmfw/exceptions.S b/pvmfw/exceptions.S
new file mode 100644
index 0000000..86ef83c
--- /dev/null
+++ b/pvmfw/exceptions.S
@@ -0,0 +1,178 @@
+/*
+ * Copyright 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Saves the volatile registers onto the stack. This currently takes 14
+ * instructions, so it can be used in exception handlers with 18 instructions
+ * left.
+ *
+ * On return, x0 and x1 are initialised to elr_el2 and spsr_el2 respectively,
+ * which can be used as the first and second arguments of a subsequent call.
+ */
+.macro save_volatile_to_stack
+ /* Reserve stack space and save registers x0-x18, x29 & x30. */
+ stp x0, x1, [sp, #-(8 * 24)]!
+ stp x2, x3, [sp, #8 * 2]
+ stp x4, x5, [sp, #8 * 4]
+ stp x6, x7, [sp, #8 * 6]
+ stp x8, x9, [sp, #8 * 8]
+ stp x10, x11, [sp, #8 * 10]
+ stp x12, x13, [sp, #8 * 12]
+ stp x14, x15, [sp, #8 * 14]
+ stp x16, x17, [sp, #8 * 16]
+ str x18, [sp, #8 * 18]
+ stp x29, x30, [sp, #8 * 20]
+
+ /*
+ * Save elr_el1 & spsr_el1. This such that we can take nested exception
+ * and still be able to unwind.
+ */
+ mrs x0, elr_el1
+ mrs x1, spsr_el1
+ stp x0, x1, [sp, #8 * 22]
+.endm
+
+/**
+ * Restores the volatile registers from the stack. This currently takes 14
+ * instructions, so it can be used in exception handlers while still leaving 18
+ * instructions left; if paired with save_volatile_to_stack, there are 4
+ * instructions to spare.
+ */
+.macro restore_volatile_from_stack
+ /* Restore registers x2-x18, x29 & x30. */
+ ldp x2, x3, [sp, #8 * 2]
+ ldp x4, x5, [sp, #8 * 4]
+ ldp x6, x7, [sp, #8 * 6]
+ ldp x8, x9, [sp, #8 * 8]
+ ldp x10, x11, [sp, #8 * 10]
+ ldp x12, x13, [sp, #8 * 12]
+ ldp x14, x15, [sp, #8 * 14]
+ ldp x16, x17, [sp, #8 * 16]
+ ldr x18, [sp, #8 * 18]
+ ldp x29, x30, [sp, #8 * 20]
+
+ /* Restore registers elr_el1 & spsr_el1, using x0 & x1 as scratch. */
+ ldp x0, x1, [sp, #8 * 22]
+ msr elr_el1, x0
+ msr spsr_el1, x1
+
+ /* Restore x0 & x1, and release stack space. */
+ ldp x0, x1, [sp], #8 * 24
+.endm
+
+/**
+ * This is a generic handler for exceptions taken at the current EL while using
+ * SP0. It behaves similarly to the SPx case by first switching to SPx, doing
+ * the work, then switching back to SP0 before returning.
+ *
+ * Switching to SPx and calling the Rust handler takes 16 instructions. To
+ * restore and return we need an additional 16 instructions, so we can implement
+ * the whole handler within the allotted 32 instructions.
+ */
+.macro current_exception_sp0 handler:req
+ msr spsel, #1
+ save_volatile_to_stack
+ bl \handler
+ restore_volatile_from_stack
+ msr spsel, #0
+ eret
+.endm
+
+/**
+ * This is a generic handler for exceptions taken at the current EL while using
+ * SPx. It saves volatile registers, calls the Rust handler, restores volatile
+ * registers, then returns.
+ *
+ * This also works for exceptions taken from EL0, if we don't care about
+ * non-volatile registers.
+ *
+ * Saving state and jumping to the Rust handler takes 15 instructions, and
+ * restoring and returning also takes 15 instructions, so we can fit the whole
+ * handler in 30 instructions, under the limit of 32.
+ */
+.macro current_exception_spx handler:req
+ save_volatile_to_stack
+ bl \handler
+ restore_volatile_from_stack
+ eret
+.endm
+
+.section .text.vector_table_el1, "ax"
+.global vector_table_el1
+.balign 0x800
+vector_table_el1:
+sync_cur_sp0:
+ current_exception_sp0 sync_exception_current
+
+.balign 0x80
+irq_cur_sp0:
+ current_exception_sp0 irq_current
+
+.balign 0x80
+fiq_cur_sp0:
+ current_exception_sp0 fiq_current
+
+.balign 0x80
+serr_cur_sp0:
+ current_exception_sp0 serr_current
+
+.balign 0x80
+sync_cur_spx:
+ current_exception_spx sync_exception_current
+
+.balign 0x80
+irq_cur_spx:
+ current_exception_spx irq_current
+
+.balign 0x80
+fiq_cur_spx:
+ current_exception_spx fiq_current
+
+.balign 0x80
+serr_cur_spx:
+ current_exception_spx serr_current
+
+.balign 0x80
+sync_lower_64:
+ current_exception_spx sync_lower
+
+.balign 0x80
+irq_lower_64:
+ current_exception_spx irq_lower
+
+.balign 0x80
+fiq_lower_64:
+ current_exception_spx fiq_lower
+
+.balign 0x80
+serr_lower_64:
+ current_exception_spx serr_lower
+
+.balign 0x80
+sync_lower_32:
+ current_exception_spx sync_lower
+
+.balign 0x80
+irq_lower_32:
+ current_exception_spx irq_lower
+
+.balign 0x80
+fiq_lower_32:
+ current_exception_spx fiq_lower
+
+.balign 0x80
+serr_lower_32:
+ current_exception_spx serr_lower