Merge pull request #14 from cqs21/opt-msr

x86_64: avoid accessing MSR registers when do trap/syscall
rcore-os · Jul 8, 2024 · 72a1a72 · 72a1a72
2 parents 0f27d58 + 4651f01
commit 72a1a72
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Showing 3 changed files with 40 additions and 46 deletions.
diff --git a/src/arch/x86_64/gdt.rs b/src/arch/x86_64/gdt.rs
@@ -63,10 +63,12 @@ pub fn init() {
         #[allow(const_item_mutation)]
         GsBase::MSR.write(tss as *const _ as u64);
 
-        Star::write_raw(
-            SegmentSelector::new(entry_count as u16 + 4, PrivilegeLevel::Ring3).0,
-            SegmentSelector::new(entry_count as u16 + 2, PrivilegeLevel::Ring0).0,
-        );
+        let sysret = SegmentSelector::new(entry_count as u16 + 4, PrivilegeLevel::Ring3).0;
+        let syscall = SegmentSelector::new(entry_count as u16 + 2, PrivilegeLevel::Ring0).0;
+        Star::write_raw(sysret, syscall);
+
+        USER_SS = sysret + 8;
+        USER_CS = sysret + 16;
     }
 }
 
@@ -81,6 +83,11 @@ unsafe fn sgdt() -> DescriptorTablePointer {
     gdt
 }
 
+#[no_mangle]
+static mut USER_SS: u16 = 0;
+#[no_mangle]
+static mut USER_CS: u16 = 0;
+
 const KCODE64: u64 = 0x00209800_00000000; // EXECUTABLE | USER_SEGMENT | PRESENT | LONG_MODE
 const UCODE64: u64 = 0x0020F800_00000000; // EXECUTABLE | USER_SEGMENT | USER_MODE | PRESENT | LONG_MODE
 const KDATA64: u64 = 0x00009200_00000000; // DATA_WRITABLE | USER_SEGMENT | PRESENT

diff --git a/src/arch/x86_64/syscall.S b/src/arch/x86_64/syscall.S
@@ -6,33 +6,28 @@ syscall_return:
     cli
 
     # save callee-saved registers
-    mov ecx, 0xC0000100
-    rdmsr
-    shl rdx, 32
-    or  rax, rdx
-    push rax                # push fsbase
     push r15
     push r14
     push r13
     push r12
     push rbp
     push rbx
 
+    # save fsbase, kernel may use
+    rdfsbase rbx
+    push rbx
+
     push rdi
     push rdi                # keep rsp 16 bytes align
     mov gs:4, rsp           # store kernel rsp -> TSS.sp0
     mov rsp, rdi            # set rsp = bottom of trap frame
 
     # pop fsbase gsbase
     swapgs                  # store kernel gsbase
-    mov ecx, 0xC0000100
-    mov edx, [rsp + 18*8+4]
-    mov eax, [rsp + 18*8]
-    wrmsr                   # pop fsbase
-    mov ecx, 0xC0000101
-    mov edx, [rsp + 19*8+4]
-    mov eax, [rsp + 19*8]
-    wrmsr                   # pop gsbase
+    mov rax, [rsp + 18*8]
+    wrfsbase rax
+    mov rax, [rsp + 19*8]
+    wrgsbase rax
 
     pop rax
     pop rbx
@@ -61,25 +56,13 @@ syscall_return:
     cmp dword ptr [rsp + 4*8], 0x100  # syscall?
     je sysret
 iret:
-    # get user cs from STAR MSR
-    mov ecx, 0xC0000081
-    rdmsr                   # msr[ecx] => edx:eax
-    shr edx, 16             # dx = user_cs32
-    lea ax, [edx + 8]       # ax = user_ss
-    add dx, 16              # dx = user_cs64
-
     # construct trap frame
-    push rax                # push ss
+    push [USER_SS]          # push ss
     push [rsp - 8*8]        # push rsp
     push [rsp + 3*8]        # push rflags
-    push rdx                # push cs
+    push [USER_CS]          # push cs
     push [rsp + 4*8]        # push rip
 
-    # recover rcx, rdx, rax
-    mov rax, [rsp - 11*8]
-    mov rcx, [rsp - 9*8]
-    mov rdx, [rsp - 8*8]
-
     iretq
 
 sysret:
@@ -137,32 +120,29 @@ trap_syscall_entry:
     push rbx
     push rax
 
-    # push fsbase gsbase
-    mov ecx, 0xC0000100
-    rdmsr
-    mov [rsp + 18*8+4], edx
-    mov [rsp + 18*8], eax
-    mov ecx, 0xC0000102     # kernelgs
-    rdmsr
-    mov [rsp + 19*8+4], edx
-    mov [rsp + 19*8], eax
+    # save user fsbase/gsbase
+    rdfsbase rbx
+    mov [rsp + 18*8], rbx
+    swapgs
+    rdgsbase rbx
+    mov [rsp + 19*8], rbx
+    swapgs
 
     # restore callee-saved registers
     mov rsp, gs:4           # load kernel rsp <- TSS.sp0
     pop rbx
     pop rbx
+
+    # restore fsbase of kernel
+    pop rbx
+    wrfsbase rbx
+
     pop rbx
     pop rbp
     pop r12
     pop r13
     pop r14
     pop r15
 
-    pop rax
-    mov ecx, 0xC0000100
-    mov rdx, rax
-    shr rdx, 32
-    wrmsr                   # pop fsbase
-
     # go back to Rust
     ret
diff --git a/src/arch/x86_64/syscall.rs b/src/arch/x86_64/syscall.rs
@@ -1,5 +1,6 @@
 use super::UserContext;
 use core::arch::global_asm;
+use x86_64::registers::control::{Cr4, Cr4Flags};
 use x86_64::registers::model_specific::{Efer, EferFlags, LStar, SFMask};
 use x86_64::registers::rflags::RFlags;
 use x86_64::VirtAddr;
@@ -18,6 +19,12 @@ pub fn init() {
             efer.insert(EferFlags::SYSTEM_CALL_EXTENSIONS);
         });
 
+        // enable `FSGSBASE` instructions
+        assert!(cpuid.get_extended_feature_info().unwrap().has_fsgsbase());
+        Cr4::update(|cr4| {
+            cr4.insert(Cr4Flags::FSGSBASE);
+        });
+
         // flags to clear on syscall
         // copy from Linux 5.0
         // TF|DF|IF|IOPL|AC|NT