[xen master] x86/efi: Skip FPU save/restore for idle vCPU in EFI runtime path

[patchbot@xxxxxxx]

commit 5740b8b0f3c7fcbd3a2fa33b47e57afc9c8bcef7
Author:     Bernhard Kaindl <bernhard.kaindl@xxxxxxxxxx>
AuthorDate: Tue Jun 16 14:15:30 2026 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Jun 16 14:15:30 2026 +0200

    x86/efi: Skip FPU save/restore for idle vCPU in EFI runtime path
    
    Anthony reported a boot-time assertion in init_xen_time() via efi_get_time()
    -> efi_rs_enter() in vcpu_save_fpu() on a Broadwell-D system:
    
      Assertion '!is_idle_vcpu(v)' failed at arch/x86/i387.c:195
    
    This became fragile after the lazy-FPU removal cleanup series:
    
    In 1792bb9a99d2 ("x86: Cleanup cr0.TS flag handling"),
    efi_rs_enter() was changed from save_fpu_enable() to vcpu_save_fpu(curr),
    which unconditionally asserts !is_idle_vcpu(v)
    so an EFI runtime call in idle context now asserts.
    
    Likewise, in dba44e051209 ("x86: Remove fully_eager_fpu"),
    efi_rs_leave() was changed to call vcpu_restore_fpu(curr),
    which has the same assertion and can fail for the same reason.
    
    Guard both EFI runtime FPU calls with !is_idle_vcpu() to skip save/restore
    for idle vCPUs, which don't have an FPU context to save/restore,
    much like the calls are guarded in __context_switch(),
    where save/restore is done only for non-idle vCPUs.
    
    Fixes: 1792bb9a99d2 ("x86: Cleanup cr0.TS flag handling")
    Fixes: dba44e051209 ("x86: Remove fully_eager_fpu")
    Reported-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
    Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
    Signed-off-by: Bernhard Kaindl <bernhard.kaindl@xxxxxxxxxx>
    Tested-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
    Acked-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
    Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
 xen/common/efi/runtime.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/xen/common/efi/runtime.c b/xen/common/efi/runtime.c
index a23fa75e37..596f2710fb 100644
--- a/xen/common/efi/runtime.c
+++ b/xen/common/efi/runtime.c
@@ -98,7 +98,8 @@ struct efi_rs_state efi_rs_enter(void)
      */
     sync_local_execstate();
     state.cr3 = read_cr3();
-    vcpu_save_fpu(current);
+    if ( !is_idle_vcpu(current) )
+        vcpu_save_fpu(current);
     asm volatile ( "fnclex; fldcw %0" :: "m" (fcw) );
     asm volatile ( "ldmxcsr %0" :: "m" (mxcsr) );
 
@@ -159,7 +160,8 @@ void efi_rs_leave(struct efi_rs_state *state)
     }
     irq_exit();
     spin_unlock(&efi_rs_lock);
-    vcpu_restore_fpu(curr);
+    if ( !is_idle_vcpu(curr) )
+        vcpu_restore_fpu(curr);
 }
 
 unsigned long efi_get_time(void)
--
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