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

[Paolo Bonzini]

On 11/03/21 16:54, Sean Christopherson wrote:
> On Tue, Feb 23, 2021, Wanpeng Li wrote:
> > 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
>
> 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
> --

I'll send this one tomorrow.

Paolo