task: Initialise syscall/sysret and add basic handlers

For running tasks at CPL-3 we need some method to transition from
CPL-0 to CPL-3. The best way to achieve this transition is using
syscall/sysret.

This commit introduces a function which is used to setup the correct
configuration in syscall MSRs to allow the use of syscall/sysret for
switching between the different privilege levels.

In addition, a syscall handler has been implemented that adds support
for terminating a user-mode task, for allowing a context switch via the
kernel schedule() function and for a simple log function. These handlers
are only an initial implementation and likely to be reworked or replaced
in a subsequent commit.

Signed-off-by: Roy Hopkins <rhopkins@suse.de>

src/svsm.rs

@@ -48,7 +48,7 @@ use svsm::sev::sev_status_init;
 use svsm::sev::utils::{rmp_adjust, RMPFlags};
 use svsm::svsm_console::SVSMIOPort;
 use svsm::svsm_paging::{init_page_table, invalidate_stage2};
-use svsm::task::{create_task, TASK_FLAG_SHARE_PT};
+use svsm::task::{create_task, init_syscall, TASK_FLAG_SHARE_PT};
 use svsm::types::{PageSize, GUEST_VMPL, PAGE_SIZE};
 use svsm::utils::{halt, immut_after_init::ImmutAfterInitCell, zero_mem_region, MemoryRegion};
 
@@ -436,6 +436,8 @@ pub extern "C" fn svsm_main(_param: u64) {
     populate_ram_fs(LAUNCH_INFO.kernel_fs_start, LAUNCH_INFO.kernel_fs_end)
         .expect("Failed to unpack FS archive");
 
+    init_syscall();
+
     ModuleLoader::enumerate().expect("Failed to initialise loadable modules");
 
     let cpus = load_acpi_cpu_info(&fw_cfg).expect("Failed to load ACPI tables");

src/task/mod.rs

@@ -5,10 +5,12 @@
 // Author: Roy Hopkins <rhopkins@suse.de>
 
 mod schedule;
+mod syscall;
 mod tasks;
 
 pub use schedule::{
     create_task, create_task_for_module, is_current_task, schedule, RunQueue, TaskNode,
     TaskPointer, TASKLIST,
 };
+pub use syscall::init_syscall;
 pub use tasks::{Task, TaskContext, TaskError, TaskState, INITIAL_TASK_ID, TASK_FLAG_SHARE_PT};

src/task/syscall.rs

@@ -0,0 +1,127 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2022-2023 SUSE LLC
+//
+// Author: Roy Hopkins <rhopkins@suse.de>
+
+use core::{
+    arch::global_asm,
+    ffi::{c_char, CStr},
+};
+
+use crate::cpu::{
+    efer::{read_efer, write_efer, EFERFlags},
+    msr::write_msr,
+    percpu::this_cpu,
+};
+use crate::types::{SVSM_CS, SVSM_USER_CS32};
+
+use super::schedule;
+
+pub enum SyscallRet {
+    Ok,
+    Unknown,
+    Terminate,
+}
+
+extern "C" {
+    static syscall_entry: u64;
+}
+
+const MSR_STAR: u32 = 0xc000_0081;
+const MSR_LSTAR: u32 = 0xc000_0082;
+const MSR_SFMASK: u32 = 0xc000_0084;
+
+#[no_mangle]
+extern "C" fn syscall_handler(index: u32, param1: u64) -> u64 {
+    let ret = match index {
+        // exit
+        0 => SyscallRet::Terminate,
+        // log
+        1 => {
+            unsafe {
+                let str = CStr::from_ptr(param1 as *const c_char);
+                log::info!("{}", str.to_str().unwrap());
+            }
+            SyscallRet::Ok
+        }
+        // sleep
+        2 => SyscallRet::Ok,
+        _ => {
+            log::info!("Invalid syscall received: {}", index);
+            SyscallRet::Unknown
+        }
+    };
+    schedule();
+    ret as u64
+}
+
+pub fn init_syscall() {
+    let mut efer = read_efer();
+    efer.insert(EFERFlags::SCE);
+    write_efer(efer);
+
+    let sysret_cs = SVSM_USER_CS32 as u64;
+    let syscall_cs = SVSM_CS as u64;
+    write_msr(MSR_STAR, sysret_cs << 48 | syscall_cs << 32);
+    unsafe {
+        write_msr(MSR_LSTAR, (&syscall_entry as *const u64) as u64);
+    }
+    // FIXME: Find correct mask for flags
+    write_msr(MSR_SFMASK, 0);
+}
+
+#[no_mangle]
+extern "C" fn get_kernel_rsp() -> u64 {
+    let task_node = this_cpu()
+        .runqueue()
+        .lock_read()
+        .current_task()
+        .expect("Invalid current task");
+    let rsp = task_node
+        .task
+        .lock_read()
+        .user
+        .as_ref()
+        .expect("Syscall from kernel task")
+        .kernel_rsp;
+    rsp
+}
+
+global_asm!(
+    r#"
+        .text
+    syscall_entry:
+        // Switch to the task kernel stack
+        push    %rcx            // User-mode return address
+
+        // Syscall arguments
+        push    %rsi
+        push    %rdi
+
+        call    get_kernel_rsp
+        pop     %rdi
+        pop     %rsi
+
+        subq    $8, %rax
+        movq    %rsp, (%rax)
+        mov     %rax, %rsp
+
+        call    syscall_handler
+
+        // Check to see if the task requested termination
+        cmp     $2, %rax            // SyscallRet::Terminate
+        jne     ret_user
+
+        addq    $8, %rsp            // Skip user mode return address
+        // Kernel stack frame should now be within launch_user_entry()
+        ret
+
+    ret_user:
+        pop     %rsp
+        pop     %rcx
+        movq    $0x202, %r11
+        sysretq
+        "#,
+    options(att_syntax)
+);