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

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;

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