KVM: PPC: Book3S HV: Don't lose pending doorbell request on migration on P9

On POWER9, when userspace reads the value of the DPDES register on a
vCPU, it is possible for 0 to be returned although there is a doorbell
interrupt pending for the vCPU.  This can lead to a doorbell interrupt
being lost across migration.  If the guest kernel uses doorbell
interrupts for IPIs, then it could malfunction because of the lost
interrupt.

This happens because a newly-generated doorbell interrupt is signalled
by setting vcpu->arch.doorbell_request to 1; the DPDES value in
vcpu->arch.vcore->dpdes is not updated, because it can only be updated
when holding the vcpu mutex, in order to avoid races.

To fix this, we OR in vcpu->arch.doorbell_request when reading the
DPDES value.

Cc: stable@vger.kernel.org # v4.13+
Fixes: 579006944e0d ("KVM: PPC: Book3S HV: Virtualize doorbell facility on POWER9")
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 36d72e9faddfbaf05220931ae9de0df9328960f9..f8975c620f41c43758bb71c1d85cdb7b75150eff 100644 (file)
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -1678,7 +1678,14 @@ static int kvmppc_get_one_reg_hv(struct kvm_vcpu *vcpu, u64 id,
                *val = get_reg_val(id, vcpu->arch.pspb);
                break;
        case KVM_REG_PPC_DPDES:
-               *val = get_reg_val(id, vcpu->arch.vcore->dpdes);
+               /*
+                * On POWER9, where we are emulating msgsndp etc.,
+                * we return 1 bit for each vcpu, which can come from
+                * either vcore->dpdes or doorbell_request.
+                * On POWER8, doorbell_request is 0.
+                */
+               *val = get_reg_val(id, vcpu->arch.vcore->dpdes |
+                                  vcpu->arch.doorbell_request);
                break;
        case KVM_REG_PPC_VTB:
                *val = get_reg_val(id, vcpu->arch.vcore->vtb);