Re: [PATCH v2 2/5] KVM: nVMX: Handle pending #DB when injecting INIT VM-exit

Sean Christopherson
2020-02-03T19:13:32Z

On Tue, Jan 28, 2020 at 01:27:12AM -0800, Oliver Upton wrote:
> SDM 27.3.4 states that the 'pending debug exceptions' VMCS field will
> be populated if a VM-exit caused by an INIT signal takes priority over a
> debug-trap. Emulate this behavior when synthesizing an INIT signal
> VM-exit into L1.
>
> Fixes: 558b8d50dbff ("KVM: x86: Fix INIT signal handling in various CPU states")
> Signed-off-by: Oliver Upton <oupton@google.com>
> ---
> arch/x86/kvm/vmx/nested.c | 23 +++++++++++++++++++++++
> 1 file changed, 23 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 95b3f4306ac2..aba16599ca69 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3572,6 +3572,27 @@ static void nested_vmx_inject_exception_vmexit(struct kvm_vcpu *vcpu,
> nested_vmx_vmexit(vcpu, EXIT_REASON_EXCEPTION_NMI, intr_info, exit_qual);
> }
>
> +static inline bool nested_vmx_check_pending_dbg(struct kvm_vcpu *vcpu)

Really dislike the name, partially because the code checks @has_payload and
partially because the part, nested_vmx_set_pending_dbg() "sets" completely
different state than this checks.

Checking has_payload may also be wrong, e.g. wouldn't it make sense to
update GUEST_PENDING_DBG_EXCEPTIONS, even if we crush it with '0'?

> +{
> + return vcpu->arch.exception.nr == DB_VECTOR &&
> + vcpu->arch.exception.pending &&
> + vcpu->arch.exception.has_payload;
> +}
> +
> +/*
> + * If a higher priority VM-exit is delivered before a debug-trap, hardware will
> + * set the 'pending debug exceptions' field appropriately for reinjection on the
> + * next VM-entry.
> + */
> +static void nested_vmx_set_pending_dbg(struct kvm_vcpu *vcpu)
> +{
> + vmcs_writel(GUEST_PENDING_DBG_EXCEPTIONS, vcpu->arch.exception.payload);
> + vcpu->arch.exception.has_payload = false;
> + vcpu->arch.exception.payload = 0;
> + vcpu->arch.exception.pending = false;
> + vcpu->arch.exception.injected = true;

This looks wrong. The #DB hasn't been injected, KVM is simply emulating
the side effect of the VMCS field being updated. E.g. KVM will have
different architecturally visible behavior depending on @has_payload.

It's not KVM's responsibility to update the pending exceptions, e.g. if
L1 wants to emulate INIT for L2 then it needs to purge the VMCS fields that
are affected by INIT.

> +}
> +
> static int vmx_check_nested_events(struct kvm_vcpu *vcpu, bool external_intr)
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -3584,6 +3605,8 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu, bool external_intr)
> test_bit(KVM_APIC_INIT, &apic->pending_events)) {
> if (block_nested_events)
> return -EBUSY;
> + if (nested_vmx_check_pending_dbg(vcpu))
> + nested_vmx_set_pending_dbg(vcpu);

I'd strongly prefer to open code the whole thing. Alternatively, if there
are other events that will need similar logic, add a single helper to take
care of everything.

E.g.

if (vcpu->arch.exception.pending &&
vcpu->arch.exception.nr == DB_VECTOR &&
vcpu->arch.exception.has_payload)
vmcs_writel(GUEST_PENDING_DBG_EXCEPTIONS,
vcpu->arch.exception.payload);

or

if (vcpu->arch.exception.pending &&
vcpu->arch.exception.nr == DB_VECTOR)
vmcs_writel(GUEST_PENDING_DBG_EXCEPTIONS,
vcpu->arch.exception.payload);

or
nested_vmx_update_guest_pending_dbg();

> clear_bit(KVM_APIC_INIT, &apic->pending_events);
> nested_vmx_vmexit(vcpu, EXIT_REASON_INIT_SIGNAL, 0, 0);
> return 0;
> --
> 2.25.0.341.g760bfbb309-goog
>