Commit a06230b6 authored by Oliver Upton's avatar Oliver Upton Committed by Paolo Bonzini
Browse files

KVM: x86: Deliver exception payload on KVM_GET_VCPU_EVENTS 



KVM allows the deferral of exception payloads when a vCPU is in guest
mode to allow the L1 hypervisor to intercept certain events (#PF, #DB)
before register state has been modified. However, this behavior is
incompatible with the KVM_{GET,SET}_VCPU_EVENTS ABI, as userspace
expects register state to have been immediately modified. Userspace may
opt-in for the payload deferral behavior with the
KVM_CAP_EXCEPTION_PAYLOAD per-VM capability. As such,
kvm_multiple_exception() will immediately manipulate guest registers if
the capability hasn't been requested.

Since the deferral is only necessary if a userspace ioctl were to be
serviced at the same as a payload bearing exception is recognized, this
behavior can be relaxed. Instead, opportunistically defer the payload
from kvm_multiple_exception() and deliver the payload before completing
a KVM_GET_VCPU_EVENTS ioctl.

Signed-off-by: default avatarOliver Upton <oupton@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 684c0422

arch/x86/kvm/x86.c

+16 −13
Original line number Diff line number Diff line
@@ -498,19 +498,7 @@ static void kvm_multiple_exception(struct kvm_vcpu *vcpu, 
		vcpu->arch.exception.error_code = error_code;

		vcpu->arch.exception.has_payload = has_payload;

		vcpu->arch.exception.payload = payload;

		/*

		 * In guest mode, payload delivery should be deferred,

		 * so that the L1 hypervisor can intercept #PF before

		 * CR2 is modified (or intercept #DB before DR6 is

		 * modified under nVMX).  However, for ABI

		 * compatibility with KVM_GET_VCPU_EVENTS and

		 * KVM_SET_VCPU_EVENTS, we can't delay payload

		 * delivery unless userspace has enabled this

		 * functionality via the per-VM capability,

		 * KVM_CAP_EXCEPTION_PAYLOAD.

		 */

		if (!vcpu->kvm->arch.exception_payload_enabled ||

		    !is_guest_mode(vcpu))

		if (!is_guest_mode(vcpu))

			kvm_deliver_exception_payload(vcpu);

		return;

	}
@@ -3803,6 +3791,21 @@ static void kvm_vcpu_ioctl_x86_get_vcpu_events(struct kvm_vcpu *vcpu, 
{

	process_nmi(vcpu);



	/*

	 * In guest mode, payload delivery should be deferred,

	 * so that the L1 hypervisor can intercept #PF before

	 * CR2 is modified (or intercept #DB before DR6 is

	 * modified under nVMX). Unless the per-VM capability,

	 * KVM_CAP_EXCEPTION_PAYLOAD, is set, we may not defer the delivery of

	 * an exception payload and handle after a KVM_GET_VCPU_EVENTS. Since we

	 * opportunistically defer the exception payload, deliver it if the

	 * capability hasn't been requested before processing a

	 * KVM_GET_VCPU_EVENTS.

	 */

	if (!vcpu->kvm->arch.exception_payload_enabled &&

	    vcpu->arch.exception.pending && vcpu->arch.exception.has_payload)

		kvm_deliver_exception_payload(vcpu);



	/*

	 * The API doesn't provide the instruction length for software

	 * exceptions, so don't report them. As long as the guest RIP

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