[PATCH] KVM: x86: remove irq disablement around KVM_SET_CLOCK/KVM_GET_CLOCK

[Marcelo Tosatti <mtosatti@redhat.com>]

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@redhat.com>

---
 
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;