Re: [PATCH v5 1/3] KVM: X86: Save&restore the triple fault request

[Sean Christopherson <seanjc@google.com>]

On Fri, Mar 18, 2022, Chenyi Qiang wrote:
> For the triple fault sythesized by KVM, e.g. the RSM path or
> nested_vmx_abort(), if KVM exits to userspace before the request is
> serviced, userspace could migrate the VM and lose the triple fault.
> Fix this issue by adding a new event KVM_VCPUEVENT_TRIPLE_FAULT in
> get/set_vcpu_events() to track the triple fault request.
> 
> Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
> ---
>  Documentation/virt/kvm/api.rst  | 6 ++++++
>  arch/x86/include/uapi/asm/kvm.h | 1 +
>  arch/x86/kvm/x86.c              | 9 ++++++++-
>  3 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 691ff84444bd..9682b0a438bd 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -1146,6 +1146,9 @@ The following bits are defined in the flags field:
>    fields contain a valid state. This bit will be set whenever
>    KVM_CAP_EXCEPTION_PAYLOAD is enabled.
>  
> +- KVM_VCPUEVENT_TRIPLE_FAULT may be set to signal that there's a
> +  triple fault request waiting to be serviced.

Please avoid "request" in the docs, as before, that's a KVM implemenation detail.
For this one, maybe "there's a pending triple fault event"?

> +
>  ARM/ARM64:
>  ^^^^^^^^^^
>  
> @@ -1241,6 +1244,9 @@ can be set in the flags field to signal that the
>  exception_has_payload, exception_payload, and exception.pending fields
>  contain a valid state and shall be written into the VCPU.
>  
> +KVM_VCPUEVENT_TRIPLE_FAULT can be set in flags field to signal that a
> +triple fault request should be made.


And here, "to signal that KVM should synthesize a triple fault for the guest"?

> +
>  ARM/ARM64:
>  ^^^^^^^^^^
>  
> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
> index bf6e96011dfe..d8ef0d993e86 100644
> --- a/arch/x86/include/uapi/asm/kvm.h
> +++ b/arch/x86/include/uapi/asm/kvm.h
> @@ -325,6 +325,7 @@ struct kvm_reinject_control {
>  #define KVM_VCPUEVENT_VALID_SHADOW	0x00000004
>  #define KVM_VCPUEVENT_VALID_SMM		0x00000008
>  #define KVM_VCPUEVENT_VALID_PAYLOAD	0x00000010
> +#define KVM_VCPUEVENT_TRIPLE_FAULT	0x00000020
>  
>  /* Interrupt shadow states */
>  #define KVM_X86_SHADOW_INT_MOV_SS	0x01
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4fa4d8269e5b..fee402a700df 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4891,6 +4891,9 @@ static void kvm_vcpu_ioctl_x86_get_vcpu_events(struct kvm_vcpu *vcpu,
>  	if (vcpu->kvm->arch.exception_payload_enabled)
>  		events->flags |= KVM_VCPUEVENT_VALID_PAYLOAD;
>  
> +	if (kvm_check_request(KVM_REQ_TRIPLE_FAULT, vcpu))
> +		events->flags |= KVM_VCPUEVENT_TRIPLE_FAULT;
> +
>  	memset(&events->reserved, 0, sizeof(events->reserved));
>  }
>  
> @@ -4903,7 +4906,8 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
>  			      | KVM_VCPUEVENT_VALID_SIPI_VECTOR
>  			      | KVM_VCPUEVENT_VALID_SHADOW
>  			      | KVM_VCPUEVENT_VALID_SMM
> -			      | KVM_VCPUEVENT_VALID_PAYLOAD))
> +			      | KVM_VCPUEVENT_VALID_PAYLOAD
> +			      | KVM_VCPUEVENT_TRIPLE_FAULT))
>  		return -EINVAL;
>  
>  	if (events->flags & KVM_VCPUEVENT_VALID_PAYLOAD) {
> @@ -4976,6 +4980,9 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
>  		}
>  	}
>  
> +	if (events->flags & KVM_VCPUEVENT_TRIPLE_FAULT)
> +		kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
> +
>  	kvm_make_request(KVM_REQ_EVENT, vcpu);

Looks correct, but this really needs a selftest, at least for the SET path since
the intent is to use that for the NOTIFY handling.  Doesn't need to be super fancy,
e.g. do port I/O from L2, inject a triple fault, and verify L1 sees the appropriate
exit.

Aha!  And for the GET path, abuse KVM_X86_SET_MCE with CR4.MCE=0 to coerce KVM into
making a KVM_REQ_TRIPLE_FAULT, that way there's no need to try and hit a timing
window to intercept the request.