[xen staging] x86/traps: Remove lazy FPU support

[patchbot@xxxxxxx]

commit 47d75569250340874e432cfc1706fbd148cecc9d
Author:     Wei Liu <wei.liu2@xxxxxxxxxx>
AuthorDate: Tue Mar 24 18:19:33 2026 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Mar 30 15:48:13 2026 +0100

    x86/traps: Remove lazy FPU support
    
    Remove lazy FPU support from the #NM exception handler used by PV
    guests since fully_eager_fpu is now always true.
    
    Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
    Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
---
 xen/arch/x86/i387.c               | 24 ------------------------
 xen/arch/x86/include/asm/i387.h   |  1 -
 xen/arch/x86/pv/misc-hypercalls.c |  3 +--
 xen/arch/x86/traps.c              | 20 +++++++++++++-------
 4 files changed, 14 insertions(+), 34 deletions(-)

diff --git a/xen/arch/x86/i387.c b/xen/arch/x86/i387.c
index 954ba3b179..7da731865f 100644
--- a/xen/arch/x86/i387.c
+++ b/xen/arch/x86/i387.c
@@ -234,30 +234,6 @@ void vcpu_restore_fpu_nonlazy(struct vcpu *v, bool 
need_stts)
         stts();
 }
 
-/* 
- * Restore FPU state when #NM is triggered.
- */
-void vcpu_restore_fpu_lazy(struct vcpu *v)
-{
-    ASSERT(!is_idle_vcpu(v));
-
-    /* Avoid recursion. */
-    clts();
-
-    if ( v->fpu_dirtied )
-        return;
-
-    ASSERT(!v->arch.fully_eager_fpu);
-
-    if ( cpu_has_xsave )
-        fpu_xrstor(v, XSTATE_LAZY);
-    else
-        fpu_fxrstor(v);
-
-    v->fpu_initialised = 1;
-    v->fpu_dirtied = 1;
-}
-
 /* 
  * On each context switch, save the necessary FPU info of VCPU being switch 
  * out. It dispatches saving operation based on CPU's capability.
diff --git a/xen/arch/x86/include/asm/i387.h b/xen/arch/x86/include/asm/i387.h
index 652d7ad2de..da0c7e945f 100644
--- a/xen/arch/x86/include/asm/i387.h
+++ b/xen/arch/x86/include/asm/i387.h
@@ -28,7 +28,6 @@ struct ix87_env {
 };
 
 void vcpu_restore_fpu_nonlazy(struct vcpu *v, bool need_stts);
-void vcpu_restore_fpu_lazy(struct vcpu *v);
 void vcpu_save_fpu(struct vcpu *v);
 void save_fpu_enable(void);
 int vcpu_init_fpu(struct vcpu *v);
diff --git a/xen/arch/x86/pv/misc-hypercalls.c 
b/xen/arch/x86/pv/misc-hypercalls.c
index 7e915d86b7..34a0717540 100644
--- a/xen/arch/x86/pv/misc-hypercalls.c
+++ b/xen/arch/x86/pv/misc-hypercalls.c
@@ -42,8 +42,7 @@ long do_fpu_taskswitch(int set)
     else
     {
         v->arch.pv.ctrlreg[0] &= ~X86_CR0_TS;
-        if ( v->fpu_dirtied )
-            clts();
+        clts();
     }
 
     return 0;
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index b6b1197697..8aa1e4181b 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2045,15 +2045,21 @@ void asmlinkage do_device_not_available(struct 
cpu_user_regs *regs)
     }
 
 #ifdef CONFIG_PV
-    vcpu_restore_fpu_lazy(curr);
-
-    if ( curr->arch.pv.ctrlreg[0] & X86_CR0_TS )
+    if ( !(curr->arch.pv.ctrlreg[0] & X86_CR0_TS) )
     {
-        pv_inject_hw_exception(X86_EXC_NM, X86_EVENT_NO_EC);
-        curr->arch.pv.ctrlreg[0] &= ~X86_CR0_TS;
+        ASSERT_UNREACHABLE();
+        domain_crash(curr->domain, "#NM but vCR0.TS clear\n");
+        return;
     }
-    else
-        TRACE_TIME(TRC_PV_MATH_STATE_RESTORE);
+
+    /*
+     * For better or worse, Xen's ABI with PV guests always clears TS on an #NM
+     * exception. Classic-xen Linux depends on this.
+     */
+    clts();
+    curr->arch.pv.ctrlreg[0] &= ~X86_CR0_TS;
+
+    pv_inject_hw_exception(X86_EXC_NM, X86_EVENT_NO_EC);
 #else
     ASSERT_UNREACHABLE();
 #endif
--
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