Re: [PATCH] x86/kvm: Fix broken irq restoration in kvm_wait

[Wanpeng Li <kernellwp@xxxxxxxxx>]

On Tue, 23 Feb 2021 at 13:25, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
>
> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>
> After commit 997acaf6b4b59c (lockdep: report broken irq restoration), the guest
> splatting below during boot:
>
>  raw_local_irq_restore() called with IRQs enabled
>  WARNING: CPU: 1 PID: 169 at kernel/locking/irqflag-debug.c:10 warn_bogus_irq_restore+0x26/0x30
>  Modules linked in: hid_generic usbhid hid
>  CPU: 1 PID: 169 Comm: systemd-udevd Not tainted 5.11.0+ #25
>  RIP: 0010:warn_bogus_irq_restore+0x26/0x30
>  Call Trace:
>   kvm_wait+0x76/0x90
>   __pv_queued_spin_lock_slowpath+0x285/0x2e0
>   do_raw_spin_lock+0xc9/0xd0
>   _raw_spin_lock+0x59/0x70
>   lockref_get_not_dead+0xf/0x50
>   __legitimize_path+0x31/0x60
>   legitimize_root+0x37/0x50
>   try_to_unlazy_next+0x7f/0x1d0
>   lookup_fast+0xb0/0x170
>   path_openat+0x165/0x9b0
>   do_filp_open+0x99/0x110
>   do_sys_openat2+0x1f1/0x2e0
>   do_sys_open+0x5c/0x80
>   __x64_sys_open+0x21/0x30
>   do_syscall_64+0x32/0x50
>   entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> The irqflags handling in kvm_wait() which ends up doing:
>
>         local_irq_save(flags);
>         safe_halt();
>         local_irq_restore(flags);
>
> which triggered a new consistency checking, we generally expect
> local_irq_save() and local_irq_restore() to be pared and sanely
> nested, and so local_irq_restore() expects to be called with
> irqs disabled.
>
> This patch fixes it by adding a local_irq_disable() after safe_halt()
> to avoid this warning.
>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---
>  arch/x86/kernel/kvm.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 5e78e01..688c84a 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -853,8 +853,10 @@ static void kvm_wait(u8 *ptr, u8 val)
>          */
>         if (arch_irqs_disabled_flags(flags))
>                 halt();
> -       else
> +       else {
>                 safe_halt();
> +               local_irq_disable();
> +       }

An alternative fix:

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 5e78e01..7127aef 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -836,12 +836,13 @@ static void kvm_kick_cpu(int cpu)

 static void kvm_wait(u8 *ptr, u8 val)
 {
-    unsigned long flags;
+    bool disabled = irqs_disabled();

     if (in_nmi())
         return;

-    local_irq_save(flags);
+    if (!disabled)
+        local_irq_disable();

     if (READ_ONCE(*ptr) != val)
         goto out;
@@ -851,13 +852,14 @@ static void kvm_wait(u8 *ptr, u8 val)
      * for irq enabled case to avoid hang when lock info is overwritten
      * in irq spinlock slowpath and no spurious interrupt occur to save us.
      */
-    if (arch_irqs_disabled_flags(flags))
+    if (disabled)
         halt();
     else
         safe_halt();

 out:
-    local_irq_restore(flags);
+    if (!disabled)
+        local_irq_enable();
 }

 #ifdef CONFIG_X86_32