Add support for reading the mxcsr registers

Signed-off-by: James Sturtevant <[email protected]>

Author: jsturtevant

src/hyperlight_host/src/hypervisor/gdb/hyperv_debug.rs

@@ -218,6 +218,7 @@ impl GuestDebug for HypervDebug {
                 fpu.xmm8, fpu.xmm9, fpu.xmm10, fpu.xmm11, fpu.xmm12, fpu.xmm13, fpu.xmm14,
                 fpu.xmm15,
             ];
+            regs.mxcsr = fpu.mxcsr;
         } else {
             log::warn!("Failed to read FPU/XMM via WHVP for debug registers");
         }

src/hyperlight_host/src/hypervisor/gdb/kvm_debug.rs

@@ -193,17 +193,30 @@ impl GuestDebug for KvmDebug {
         regs.rip = vcpu_regs.rip;
         regs.rflags = vcpu_regs.rflags;
 
-         // Read XMM registers from FPU state
+        // Read XMM registers from FPU state
+        // note kvm get_fpu doesn't actually set or read the mxcsr value
+        // https://elixir.bootlin.com/linux/v6.16/source/arch/x86/kvm/x86.c#L12229
         match vcpu_fd.get_fpu() {
             Ok(fpu) => {
                 // Convert KVM XMM registers ([u8; 16] x 16) to [u128; 16]
-                regs.xmm = fpu.xmm.map(u128::from_le_bytes);;
+                regs.xmm = fpu.xmm.map(u128::from_le_bytes);
             },
             Err(e) => {
                 log::warn!("Failed to read FPU state for XMM registers: {:?}", e);
             }
         }
 
+        // Read MXCSR from XSAVE (MXCSR is at byte offset 24 -> u32 index 6)
+        // Todo maybe I could use xsave to read the registers too instead of a separate call to get_fpu
+        match vcpu_fd.get_xsave() {
+            Ok(xsave) => {
+                regs.mxcsr = xsave.region[6];
+            }
+            Err(e) => {
+                log::warn!("Failed to read XSAVE for MXCSR: {:?}", e);
+            }
+        }
+
         Ok(())
     }
 

src/hyperlight_host/src/hypervisor/gdb/mod.rs

@@ -110,6 +110,7 @@ pub(crate) struct X86_64Regs {
     pub(crate) rip: u64,
     pub(crate) rflags: u64,
     pub(crate) xmm: [u128; 16],
+    pub(crate) mxcsr: u32,
 }
 
 /// Defines the possible reasons for which a vCPU can be stopped when debugging

src/hyperlight_host/src/hypervisor/gdb/mshv_debug.rs

@@ -225,6 +225,7 @@ impl GuestDebug for MshvDebug {
             Ok(fpu) => {
                 // MSHV exposes XMM as [[u8; 16]; 16]. Convert to [u128; 16]
                 regs.xmm = fpu.xmm.map(u128::from_le_bytes);
+                regs.mxcsr = fpu.mxcsr;
             }
             Err(e) => {
                 log::warn!("Failed to read FPU state for XMM registers (MSHV): {:?}", e);
@@ -269,8 +270,6 @@ impl GuestDebug for MshvDebug {
 
             rip: regs.rip,
             rflags: regs.rflags,
-
-            
         };
 
         vcpu_fd

src/hyperlight_host/src/hypervisor/gdb/x86_64_target.rs

@@ -228,6 +228,7 @@ impl SingleThreadBase for HyperlightSandboxTarget {
                 regs.rip = read_regs.rip;
                 regs.eflags = read_regs.rflags as u32;
                 regs.xmm = read_regs.xmm;
+                regs.mxcsr = read_regs.mxcsr;
 
                 Ok(())
             }
@@ -269,6 +270,7 @@ impl SingleThreadBase for HyperlightSandboxTarget {
             rip: regs.rip,
             rflags: u64::from(regs.eflags),
             xmm: regs.xmm,
+            mxcsr: regs.mxcsr,
         };
 
         match self.send_command(DebugMsg::WriteRegisters(regs))? {

src/hyperlight_host/src/hypervisor/kvm.rs

@@ -587,6 +587,9 @@ impl Hypervisor for KVMDriver {
             mxcsr: MXCSR_DEFAULT,
             ..Default::default() // zero out the rest
         };
+
+        // note kvm set_fpu doesn't actually set or read the mxcsr value
+        // https://elixir.bootlin.com/linux/v6.16/source/arch/x86/kvm/x86.c#L12229
         self.vcpu_fd.set_fpu(&fpu)?;
 
         // run