The disablement of interrupts at KVM_SET_CLOCK/KVM_GET_CLOCK
attempts to disable interrupts in that section to protect
the values that are calculated in that section from interrupt interference.
now_ns is calculated inside the irq protected region,
user_ns.clock is passed from userspace (therefore not susceptible
to interrupt variation).
About the line
now_ns = __get_kvmclock_ns(kvm); (1)
Interrupts can happen afterwards local_irq_enable(),
rendering "now_ns" relative to its execution time PLUS
interrupt time.
Therefore the local_irq_disable() / local_irq_enable() protection is not
necessary (that is: interrupts triggering after local_irq_enable cause
the same problem that the protection is trying to avoid).
With this reasoning, and the -RT bug that the irq disablement causes
(because spin_lock is now a sleeping lock), remove the IRQ protection as it causes:
[ 1064.668109] in_atomic(): 0, irqs_disabled(): 1, pid: 15296, name:m
[ 1064.668110] INFO: lockdep is turned off.
[ 1064.668110] irq event stamp: 0
[ 1064.668112] hardirqs last enabled at (0): [< (null)>] )
[ 1064.668116] hardirqs last disabled at (0): [<ffffffff9308184a>] c0
[ 1064.668118] softirqs last enabled at (0): [<ffffffff9308184a>] c0
[ 1064.668118] softirqs last disabled at (0): [< (null)>] )
[ 1064.668121] CPU: 13 PID: 15296 Comm: qemu-kvm Not tainted 3.10.0-1
[ 1064.668121] Hardware name: Dell Inc. PowerEdge R730/0H21J3, BIOS 5
[ 1064.668123] ffff8c1796b88000 00000000afe7344c ffff8c179abf3c68 f3
[ 1064.668125] ffff8c179abf3c90 ffffffff930ccb3d ffff8c1b992b3610 f0
[ 1064.668126] 00007ffc1a26fbc0 ffff8c179abf3cb0 ffffffff9375f694 f0
[ 1064.668126] Call Trace:
[ 1064.668132] [<ffffffff93757413>] dump_stack+0x19/0x1b
[ 1064.668135] [<ffffffff930ccb3d>] __might_sleep+0x12d/0x1f0
[ 1064.668138] [<ffffffff9375f694>] rt_spin_lock+0x24/0x60
[ 1064.668155] [<ffffffffc06ab996>] __get_kvmclock_ns+0x36/0x110 [k]
[ 1064.668159] [<ffffffff93112993>] ? futex_wait_queue_me+0x103/0x10
[ 1064.668171] [<ffffffffc06b8782>] kvm_arch_vm_ioctl+0xa2/0xd70 [k]
[ 1064.668173] [<ffffffff9311333c>] ? futex_wait+0x1ac/0x2a0
On -RT kernels.
Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
---
arch/x86/kvm/x86.c | 4 ----
1 file changed, 4 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7c39b8a..935143b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4062,10 +4062,8 @@ long kvm_arch_vm_ioctl(struct file *filp,
goto out;
r = 0;
- local_irq_disable();
now_ns = __get_kvmclock_ns(kvm);
kvm->arch.kvmclock_offset += user_ns.clock - now_ns;
- local_irq_enable();
kvm_gen_update_masterclock(kvm);
break;
}
@@ -4073,11 +4071,9 @@ long kvm_arch_vm_ioctl(struct file *filp,
struct kvm_clock_data user_ns;
u64 now_ns;
- local_irq_disable();
now_ns = __get_kvmclock_ns(kvm);
user_ns.clock = now_ns;
user_ns.flags = kvm->arch.use_master_clock ? KVM_CLOCK_TSC_STABLE : 0;
- local_irq_enable();
memset(&user_ns.pad, 0, sizeof(user_ns.pad));
r = -EFAULT;
