Re: [PATCH v6 1/3] KVM: X86: Save&restore the triple fault request

[Chenyi Qiang]

Thanks Sean for your review!

On 5/19/2022 2:42 AM, Sean Christopherson wrote:
> Nits on the shortlog...
>
> Please don't capitalize x86, spell out "and" instead of using an ampersand (though
> I think it can be omitted entirely), and since there are plenty of chars left, call
> out that this is adding/extending KVM's ABI, e.g. it's not obvious from the shortlog
> where/when the save+restore happens.
>
>    KVM: x86: Extend KVM_{G,S}ET_VCPU_EVENTS to support pending triple fault

Will fix it.

> On Thu, Apr 21, 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.
> >
> > Add the support to save and restore the triple fault event from
> > userspace. Introduce a new event KVM_VCPUEVENT_VALID_TRIPLE_FAULT in
> > get/set_vcpu_events to track the triple fault request.
> >
> > Note that in the set_vcpu_events path, userspace is able to set/clear
> > the triple fault request through triple_fault_pending field.
> >
> > Signed-off-by: Chenyi Qiang <chenyi.qiang@xxxxxxxxx>
> > ---
> >   Documentation/virt/kvm/api.rst  |  7 +++++++
> >   arch/x86/include/uapi/asm/kvm.h |  4 +++-
> >   arch/x86/kvm/x86.c              | 15 +++++++++++++--
> >   3 files changed, 23 insertions(+), 3 deletions(-)
> >
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index 72183ae628f7..e09ce3cb49c5 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -1150,6 +1150,9 @@ 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_VALID_TRIPLE_FAULT may be set to signal that the
> > +  triple_fault_pending field contains a valid state.
> > +
> >   ARM64:
> >   ^^^^^^
> >   
> > @@ -1245,6 +1248,10 @@ 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_VALID_TRIPLE_FAULT can be set in flags field to signal that
> > +the triple_fault_pending field contains a valid state and shall be written
> > +into the VCPU.
> > +
> >   ARM64:
> >   ^^^^^^
> >   
> > diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
> > index 21614807a2cb..fd083f6337af 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_VALID_TRIPLE_FAULT	0x00000020
> >   
> >   /* Interrupt shadow states */
> >   #define KVM_X86_SHADOW_INT_MOV_SS	0x01
> > @@ -359,7 +360,8 @@ struct kvm_vcpu_events {
> >   		__u8 smm_inside_nmi;
> >   		__u8 latched_init;
> >   	} smi;
> > -	__u8 reserved[27];
> > +	__u8 triple_fault_pending;
>
> What about writing this as
>
> 	struct {
> 		__u8 pending;
> 	} triple_fault;
>
> to match the other events?  It's kinda silly, but I find it easier to visually
> identify the various events that are handled by kvm_vcpu_events.

Sure, will change in this format.

> > +	__u8 reserved[26];
> >   	__u8 exception_has_payload;
> >   	__u64 exception_payload;
> >   };
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index ab336f7c82e4..c8b9b0bc42aa 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -4911,9 +4911,12 @@ static void kvm_vcpu_ioctl_x86_get_vcpu_events(struct kvm_vcpu *vcpu,
> >   		!!(vcpu->arch.hflags & HF_SMM_INSIDE_NMI_MASK);
> >   	events->smi.latched_init = kvm_lapic_latched_init(vcpu);
> >   
> > +	events->triple_fault_pending = kvm_test_request(KVM_REQ_TRIPLE_FAULT, vcpu);
> > +
> >   	events->flags = (KVM_VCPUEVENT_VALID_NMI_PENDING
> >   			 | KVM_VCPUEVENT_VALID_SHADOW
> > -			 | KVM_VCPUEVENT_VALID_SMM);
> > +			 | KVM_VCPUEVENT_VALID_SMM
> > +			 | KVM_VCPUEVENT_VALID_TRIPLE_FAULT);
>
> Does setting KVM_VCPUEVENT_VALID_TRIPLE_FAULT need to be guarded with a capability,
> a la KVM_CAP_EXCEPTION_PAYLOAD, so that migrating from a new KVM to an old KVM doesn't
> fail?  Seems rather pointless since the VM is likely hosed either way...

Indeed, at least adding a capability makes it more compatible. Will add 
it in next version.

> >   	if (vcpu->kvm->arch.exception_payload_enabled)
> >   		events->flags |= KVM_VCPUEVENT_VALID_PAYLOAD;