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Commit 36ee41d1 authored by Paul Mackerras's avatar Paul Mackerras
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KVM: PPC: Book3S HV: Drop locks before reading guest memory 



Running with CONFIG_DEBUG_ATOMIC_SLEEP reveals that HV KVM tries to
read guest memory, in order to emulate guest instructions, while
preempt is disabled and a vcore lock is held.  This occurs in
kvmppc_handle_exit_hv(), called from post_guest_process(), when
emulating guest doorbell instructions on POWER9 systems, and also
when checking whether we have hit a hypervisor breakpoint.
Reading guest memory can cause a page fault and thus cause the
task to sleep, so we need to avoid reading guest memory while
holding a spinlock or when preempt is disabled.

To fix this, we move the preempt_enable() in kvmppc_run_core() to
before the loop that calls post_guest_process() for each vcore that
has just run, and we drop and re-take the vcore lock around the calls
to kvmppc_emulate_debug_inst() and kvmppc_emulate_doorbell_instr().

Dropping the lock is safe with respect to the iteration over the
runnable vcpus in post_guest_process(); for_each_runnable_thread
is actually safe to use locklessly.  It is possible for a vcpu
to become runnable and add itself to the runnable_threads array
(code near the beginning of kvmppc_run_vcpu()) and then get included
in the iteration in post_guest_process despite the fact that it
has not just run.  This is benign because vcpu->arch.trap and
vcpu->arch.ceded will be zero.

Cc: stable@vger.kernel.org # v4.13+
Fixes: 57900694 ("KVM: PPC: Book3S HV: Virtualize doorbell facility on POWER9")
Signed-off-by: default avatarPaul Mackerras <paulus@ozlabs.org>
parent 9b9b13a6

arch/powerpc/kvm/book3s_hv.c

+12 −4
Original line number Diff line number Diff line
@@ -1008,8 +1008,6 @@ static int kvmppc_emulate_doorbell_instr(struct kvm_vcpu *vcpu) 
	struct kvm *kvm = vcpu->kvm;

	struct kvm_vcpu *tvcpu;



	if (!cpu_has_feature(CPU_FTR_ARCH_300))

		return EMULATE_FAIL;

	if (kvmppc_get_last_inst(vcpu, INST_GENERIC, &inst) != EMULATE_DONE)

		return RESUME_GUEST;

	if (get_op(inst) != 31)
@@ -1059,6 +1057,7 @@ static int kvmppc_emulate_doorbell_instr(struct kvm_vcpu *vcpu) 
	return RESUME_GUEST;

}



/* Called with vcpu->arch.vcore->lock held */

static int kvmppc_handle_exit_hv(struct kvm_run *run, struct kvm_vcpu *vcpu,

				 struct task_struct *tsk)

{
@@ -1179,7 +1178,10 @@ static int kvmppc_handle_exit_hv(struct kvm_run *run, struct kvm_vcpu *vcpu, 
				swab32(vcpu->arch.emul_inst) :

				vcpu->arch.emul_inst;

		if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP) {

			/* Need vcore unlocked to call kvmppc_get_last_inst */

			spin_unlock(&vcpu->arch.vcore->lock);

			r = kvmppc_emulate_debug_inst(run, vcpu);

			spin_lock(&vcpu->arch.vcore->lock);

		} else {

			kvmppc_core_queue_program(vcpu, SRR1_PROGILL);

			r = RESUME_GUEST;
@@ -1194,8 +1196,13 @@ static int kvmppc_handle_exit_hv(struct kvm_run *run, struct kvm_vcpu *vcpu, 
	 */

	case BOOK3S_INTERRUPT_H_FAC_UNAVAIL:

		r = EMULATE_FAIL;

		if ((vcpu->arch.hfscr >> 56) == FSCR_MSGP_LG)

		if (((vcpu->arch.hfscr >> 56) == FSCR_MSGP_LG) &&

		    cpu_has_feature(CPU_FTR_ARCH_300)) {

			/* Need vcore unlocked to call kvmppc_get_last_inst */

			spin_unlock(&vcpu->arch.vcore->lock);

			r = kvmppc_emulate_doorbell_instr(vcpu);

			spin_lock(&vcpu->arch.vcore->lock);

		}

		if (r == EMULATE_FAIL) {

			kvmppc_core_queue_program(vcpu, SRR1_PROGILL);

			r = RESUME_GUEST;
@@ -2946,13 +2953,14 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc) 
	/* make sure updates to secondary vcpu structs are visible now */

	smp_mb();



	preempt_enable();



	for (sub = 0; sub < core_info.n_subcores; ++sub) {

		pvc = core_info.vc[sub];

		post_guest_process(pvc, pvc == vc);

	}



	spin_lock(&vc->lock);

	preempt_enable();



 out:

	vc->vcore_state = VCORE_INACTIVE;