x86: Add support for maintaining the x87 tag word

The current implementation of the x87 never updates the x87 tag
word. This is currently not a big issue since the simulated x87 never
checks for stack overflows, however this becomes an issue when
switching between a virtualized CPU and a simulated CPU. This
changeset adds support, which is enabled by default, for updating the
tag register to every floating point microop that updates the stack
top using the spm mechanism.

The new tag words is generated by the helper function
X86ISA::genX87Tags(). This function is currently limited to flagging a
stack position as valid or invalid and does not try to distinguish
between the valid, zero, and special states.

1 files changed, 20 insertions, 0 deletions

diff --git a/src/arch/x86/utility.cc b/src/arch/x86/utility.cc
index 2398ca073..3df948986 100644
--- a/src/arch/x86/utility.cc
+++ b/src/arch/x86/utility.cc
@@ -268,4 +268,24 @@ setRFlags(ThreadContext *tc, uint64_t val)
     tc->setMiscReg(MISCREG_RFLAGS, val & ~(ccFlagMask | cfofMask | DFBit));
 }
 
+uint16_t
+genX87Tags(uint16_t ftw, uint8_t top, int8_t spm)
+{
+    const uint8_t new_top((top + spm + 8) % 8);
+
+    if (spm > 0) {
+        // Removing elements from the stack. Flag the elements as empty.
+        for (int i = top; i != new_top; i = (i + 1 + 8) % 8)
+            ftw |= 0x3 << (2 * i);
+    } else if (spm < 0) {
+        // Adding elements to the stack. Flag the new elements as
+        // valid. We should ideally decode them and "do the right
+        // thing".
+        for (int i = new_top; i != top; i = (i + 1 + 8) % 8)
+            ftw &= ~(0x3 << (2 * i));
+    }
+
+    return ftw;
+}
+
 } // namespace X86_ISA