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

[Sean Christopherson <seanjc@google.com>]

On Tue, Feb 23, 2021, Wanpeng Li wrote:
> On Tue, 23 Feb 2021 at 13:25, Wanpeng Li <kernellwp@gmail.com> wrote:
> >
> > From: Wanpeng Li <wanpengli@tencent.com>
> >
> > 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@arm.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> > ---
> >  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

A third option would be to split the paths.  In the end, it's only the ptr/val
line that's shared.

---
 arch/x86/kernel/kvm.c | 23 ++++++++++-------------
 1 file changed, 10 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 5e78e01ca3b4..78bb0fae3982 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -836,28 +836,25 @@ static void kvm_kick_cpu(int cpu)

 static void kvm_wait(u8 *ptr, u8 val)
 {
-	unsigned long flags;
-
 	if (in_nmi())
 		return;

-	local_irq_save(flags);
-
-	if (READ_ONCE(*ptr) != val)
-		goto out;
-
 	/*
 	 * halt until it's our turn and kicked. Note that we do safe halt
 	 * 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))
-		halt();
-	else
-		safe_halt();
+	if (irqs_disabled()) {
+		if (READ_ONCE(*ptr) == val)
+			halt();
+	} else {
+		local_irq_disable();

-out:
-	local_irq_restore(flags);
+		if (READ_ONCE(*ptr) == val)
+			safe_halt();
+
+		local_irq_enable();
+	}
 }

 #ifdef CONFIG_X86_32
--