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

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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




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