Kernel: Use FXSR feature only if supported by CPU

If FXSR is not present, fall back to fnsave and frstor instructions.
These instructions aren't available on the canonical i686 CPU which is
the Pentium Pro processor.

1 files changed, 23 insertions, 6 deletions

diff --git a/Kernel/Arch/i386/CPU.cpp b/Kernel/Arch/i386/CPU.cpp
index 24bf137685..4cb3533fd0 100644
--- a/Kernel/Arch/i386/CPU.cpp
+++ b/Kernel/Arch/i386/CPU.cpp
@@ -1049,8 +1049,12 @@ UNMAP_AFTER_INIT void Processor::cpu_setup()
     //       initialized yet!
     cpu_detect();
 
-    if (has_feature(CPUFeature::SSE))
+    if (has_feature(CPUFeature::SSE)) {
+        // enter_thread_context() assumes that if a x86 CPU supports SSE then it also supports FXSR.
+        // SSE support without FXSR is an extremely unlikely scenario, so let's be pragmatic about it.
+        VERIFY(has_feature(CPUFeature::FXSR));
         sse_init();
+    }
 
     write_cr0(read_cr0() | 0x00010000);
 
@@ -1233,8 +1237,12 @@ UNMAP_AFTER_INIT void Processor::initialize(u32 cpu)
     if (cpu == 0) {
         VERIFY((FlatPtr(&s_clean_fpu_state) & 0xF) == 0);
         asm volatile("fninit");
-        asm volatile("fxsave %0"
-                     : "=m"(s_clean_fpu_state));
+        if (has_feature(CPUFeature::FXSR))
+            asm volatile("fxsave %0"
+                         : "=m"(s_clean_fpu_state));
+        else
+            asm volatile("fnsave %0"
+                         : "=m"(s_clean_fpu_state));
     }
 
     m_info = new ProcessorInfo(*this);
@@ -1412,12 +1420,18 @@ extern "C" void enter_thread_context(Thread* from_thread, Thread* to_thread)
     VERIFY(from_thread == to_thread || from_thread->state() != Thread::Running);
     VERIFY(to_thread->state() == Thread::Running);
 
+    bool has_fxsr = Processor::current().has_feature(CPUFeature::FXSR);
     Processor::set_current_thread(*to_thread);
 
     auto& from_tss = from_thread->tss();
     auto& to_tss = to_thread->tss();
-    asm volatile("fxsave %0"
-                 : "=m"(from_thread->fpu_state()));
+
+    if (has_fxsr)
+        asm volatile("fxsave %0"
+                     : "=m"(from_thread->fpu_state()));
+    else
+        asm volatile("fnsave %0"
+                     : "=m"(from_thread->fpu_state()));
 
     from_tss.fs = get_fs();
     from_tss.gs = get_gs();
@@ -1444,7 +1458,10 @@ extern "C" void enter_thread_context(Thread* from_thread, Thread* to_thread)
     to_thread->set_cpu(processor.get_id());
     processor.restore_in_critical(to_thread->saved_critical());
 
-    asm volatile("fxrstor %0" ::"m"(to_thread->fpu_state()));
+    if (has_fxsr)
+        asm volatile("fxrstor %0" ::"m"(to_thread->fpu_state()));
+    else
+        asm volatile("frstor %0" ::"m"(to_thread->fpu_state()));
 
     // TODO: ioperm?
 }