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

    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@intel.com>
    > ---
    > 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.

    > + __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...

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

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