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KVM: race-free exit from KVM_RUN without POSIX signals

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The purpose of the KVM_SET_SIGNAL_MASK API is to let userspace "kick"
a VCPU out of KVM_RUN through a POSIX signal.  A signal is attached
to a dummy signal handler; by blocking the signal outside KVM_RUN and
unblocking it inside, this possible race is closed:

          VCPU thread                     service thread
   --------------------------------------------------------------
        check flag
                                          set flag
                                          raise signal
        (signal handler does nothing)
        KVM_RUN

However, one issue with KVM_SET_SIGNAL_MASK is that it has to take
tsk->sighand->siglock on every KVM_RUN.  This lock is often on a
remote NUMA node, because it is on the node of a thread's creator.
Taking this lock can be very expensive if there are many userspace
exits (as is the case for SMP Windows VMs without Hyper-V reference
time counter).

As an alternative, we can put the flag directly in kvm_run so that
KVM can see it:

          VCPU thread                     service thread
   --------------------------------------------------------------
                                          raise signal
        signal handler
          set run->immediate_exit
        KVM_RUN
          check run->immediate_exit

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini
2017-02-08 11:50:15 +01:00
parent bbd6411513
commit 460df4c1fc
7 changed files with 39 additions and 5 deletions
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4
include/uapi/linux/kvm.h
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@@ -218,7 +218,8 @@ struct kvm_hyperv_exit {
struct kvm_run {
/* in */
__u8 request_interrupt_window;
__u8 padding1[7];
__u8 immediate_exit;
__u8 padding1[6];
/* out */
__u32 exit_reason;
@@ -881,6 +882,7 @@ struct kvm_ppc_resize_hpt {
#define KVM_CAP_SPAPR_RESIZE_HPT 133
#define KVM_CAP_PPC_MMU_RADIX 134
#define KVM_CAP_PPC_MMU_HASH_V3 135
#define KVM_CAP_IMMEDIATE_EXIT 136
#ifdef KVM_CAP_IRQ_ROUTING