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KVM: use KVM_CAP_IMMEDIATE_EXIT

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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).

KVM_CAP_IMMEDIATE_EXIT provides an alternative, where the flag is
placed 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

The previous patches changed QEMU so that the only blocked signal is
SIG_IPI, so we can now stop using KVM_SET_SIGNAL_MASK and sigtimedwait
if KVM_CAP_IMMEDIATE_EXIT is available.

On a 14-VCPU guest, an "inl" operation goes down from 30k to 6k on
an unlocked (no BQL) MemoryRegion, or from 30k to 15k if the BQL
is involved.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2017-02-08 13:52:50 +01:00
parent c5c6679d37
commit cf0f7cf903
1 changed files with 42 additions and 4 deletions
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46
kvm-all.c
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@ -120,6 +120,7 @@ bool kvm_vm_attributes_allowed;
bool kvm_direct_msi_allowed;
bool kvm_ioeventfd_any_length_allowed;
bool kvm_msi_use_devid;
static bool kvm_immediate_exit;
static const KVMCapabilityInfo kvm_required_capabilites[] = {
KVM_CAP_INFO(USER_MEMORY),
@ -1619,6 +1620,7 @@ static int kvm_init(MachineState *ms)
goto err;
}
kvm_immediate_exit = kvm_check_extension(s, KVM_CAP_IMMEDIATE_EXIT);
s->nr_slots = kvm_check_extension(s, KVM_CAP_NR_MEMSLOTS);
/* If unspecified, use the default value */
@ -1899,6 +1901,20 @@ static __thread int pending_sigbus_code;
static __thread bool have_sigbus_pending;
#endif
static void kvm_cpu_kick(CPUState *cpu)
{
atomic_set(&cpu->kvm_run->immediate_exit, 1);
}
static void kvm_cpu_kick_self(void)
{
if (kvm_immediate_exit) {
kvm_cpu_kick(current_cpu);
} else {
qemu_cpu_kick_self();
}
}
static void kvm_eat_signals(CPUState *cpu)
{
struct timespec ts = { 0, 0 };
@ -1907,6 +1923,15 @@ static void kvm_eat_signals(CPUState *cpu)
sigset_t chkset;
int r;
if (kvm_immediate_exit) {
atomic_set(&cpu->kvm_run->immediate_exit, 0);
/* Write kvm_run->immediate_exit before the cpu->exit_request
* write in kvm_cpu_exec.
*/
smp_wmb();
return;
}
sigemptyset(&waitset);
sigaddset(&waitset, SIG_IPI);
@ -1955,9 +1980,14 @@ int kvm_cpu_exec(CPUState *cpu)
* instruction emulation. This self-signal will ensure that we
* leave ASAP again.
*/
qemu_cpu_kick_self();
kvm_cpu_kick_self();
}
/* Read cpu->exit_request before KVM_RUN reads run->immediate_exit.
* Matching barrier in kvm_eat_signals.
*/
smp_rmb();
run_ret = kvm_vcpu_ioctl(cpu, KVM_RUN, 0);
attrs = kvm_arch_post_run(cpu, run);
@ -2431,8 +2461,12 @@ static int kvm_set_signal_mask(CPUState *cpu, const sigset_t *sigset)
return r;
}
static void dummy_signal(int sig)
static void kvm_ipi_signal(int sig)
{
if (current_cpu) {
assert(kvm_immediate_exit);
kvm_cpu_kick(current_cpu);
}
}
void kvm_init_cpu_signals(CPUState *cpu)
@ -2442,7 +2476,7 @@ void kvm_init_cpu_signals(CPUState *cpu)
struct sigaction sigact;
memset(&sigact, 0, sizeof(sigact));
sigact.sa_handler = dummy_signal;
sigact.sa_handler = kvm_ipi_signal;
sigaction(SIG_IPI, &sigact, NULL);
pthread_sigmask(SIG_BLOCK, NULL, &set);
@ -2451,7 +2485,11 @@ void kvm_init_cpu_signals(CPUState *cpu)
pthread_sigmask(SIG_SETMASK, &set, NULL);
#endif
sigdelset(&set, SIG_IPI);
r = kvm_set_signal_mask(cpu, &set);
if (kvm_immediate_exit) {
r = pthread_sigmask(SIG_SETMASK, &set, NULL);
} else {
r = kvm_set_signal_mask(cpu, &set);
}
if (r) {
fprintf(stderr, "kvm_set_signal_mask: %s\n", strerror(-r));
exit(1);