Re: [PATCH 11/16] KVM: TDX: Always block INIT/SIPI

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 1/8/2025 10:40 PM, Sean Christopherson wrote:
> On Wed, Jan 08, 2025, Binbin Wu wrote:
>> On 1/8/2025 3:21 PM, Xiaoyao Li wrote:
>>> On 12/9/2024 9:07 AM, Binbin Wu wrote:
> 
> ...
> 
>>>> ---
>>>>    arch/x86/kvm/lapic.c    |  2 +-
>>>>    arch/x86/kvm/vmx/main.c | 19 ++++++++++++++++++-
>>>>    2 files changed, 19 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>>>> index 474e0a7c1069..f93c382344ee 100644
>>>> --- a/arch/x86/kvm/lapic.c
>>>> +++ b/arch/x86/kvm/lapic.c
>>>> @@ -3365,7 +3365,7 @@ int kvm_apic_accept_events(struct kvm_vcpu *vcpu)
>>>>          if (test_and_clear_bit(KVM_APIC_INIT, &apic->pending_events)) {
>>>>            kvm_vcpu_reset(vcpu, true);
>>>> -        if (kvm_vcpu_is_bsp(apic->vcpu))
>>>> +        if (kvm_vcpu_is_bsp(vcpu))
>>>>                vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
>>>>            else
>>>>                vcpu->arch.mp_state = KVM_MP_STATE_INIT_RECEIVED;
>>>> diff --git a/arch/x86/kvm/vmx/main.c b/arch/x86/kvm/vmx/main.c
>>>> index 8ec96646faec..7f933f821188 100644
>>>> --- a/arch/x86/kvm/vmx/main.c
>>>> +++ b/arch/x86/kvm/vmx/main.c
>>>> @@ -115,6 +115,11 @@ static void vt_vcpu_free(struct kvm_vcpu *vcpu)
>>>>      static void vt_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>>>>    {
>>>> +    /*
>>>> +     * TDX has its own sequence to do init during TD build time (by
>>>> +     * KVM_TDX_INIT_VCPU) and it doesn't support INIT event during TD
>>>> +     * runtime.
>>>> +     */
>>>
>>> The first half is confusing. It seems to mix up init(ialization) with INIT
>>> event.
>>>
>>> And this callback is about *reset*, which can be due to INIT event or not.
>>> That's why it has a second parameter of init_event. The comment needs to
>>> clarify why reset is not needed for both cases.
>>>
>>> I think we can just say TDX doesn't support vcpu reset no matter due to
>>> INIT event or not.
> 
> That's not entirely accurate either though.  TDX does support KVM's version of
> RESET, because KVM's RESET is "power-on", i.e. vCPU creation.  Emulation of
> runtime RESET is userspace's responsibility.
> 
> The real reason why KVM doesn't do anything during KVM's RESET is that what
> little setup KVM does/can do needs to be defered until after guest CPUID is
> configured.

It's much clear now.

> KVM should also WARN if a TDX vCPU gets INIT, no?

Agreed.

> Side topic, the comment about x2APIC in tdx_vcpu_init() is too specific, e.g.
> calling out that x2APIC support is enumerated in CPUID.0x1.ECX isn't necessary,
> and stating that userspace must use KVM_SET_CPUID2 is flat out wrong.  Very
> technically, KVM_SET_CPUID is also a valid option, it's just not used in practice
> because it doesn't support setting non-zero indices (but in theory it could be
> used to enable x2APIC).
> 
> E.g. something like this?

LGTM.

> diff --git a/arch/x86/kvm/vmx/main.c b/arch/x86/kvm/vmx/main.c
> index d2e78e6675b9..e36fba94fa14 100644
> --- a/arch/x86/kvm/vmx/main.c
> +++ b/arch/x86/kvm/vmx/main.c
> @@ -115,13 +115,10 @@ static void vt_vcpu_free(struct kvm_vcpu *vcpu)
>   
>   static void vt_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>   {
> -       /*
> -        * TDX has its own sequence to do init during TD build time (by
> -        * KVM_TDX_INIT_VCPU) and it doesn't support INIT event during TD
> -        * runtime.
> -        */
> -       if (is_td_vcpu(vcpu))
> +       if (is_td_vcpu(vcpu)) {
> +               tdx_vcpu_reset(vcpu, init_event);
>                  return;
> +       }
>   
>          vmx_vcpu_reset(vcpu, init_event);
>   }
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 9e490fccf073..a587f59167a7 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -2806,9 +2806,8 @@ static int tdx_vcpu_init(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *cmd)
>                  return -EINVAL;
>   
>          /*
> -        * As TDX requires X2APIC, set local apic mode to X2APIC.  User space
> -        * VMM, e.g. qemu, is required to set CPUID[0x1].ecx.X2APIC=1 by
> -        * KVM_SET_CPUID2.  Otherwise kvm_apic_set_base() will fail.
> +        * TDX requires x2APIC, userspace is responsible for configuring guest
> +        * CPUID accordingly.
>           */
>          apic_base = APIC_DEFAULT_PHYS_BASE | LAPIC_MODE_X2APIC |
>                  (kvm_vcpu_is_reset_bsp(vcpu) ? MSR_IA32_APICBASE_BSP : 0);
> @@ -2827,6 +2826,19 @@ static int tdx_vcpu_init(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *cmd)
>          return 0;
>   }
>   
> +void tdx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> +{
> +       /*
> +        * Yell on INIT, as TDX doesn't support INIT, i.e. KVM should drop all
> +        * INIT events.
> +        *
> +        * Defer initializing vCPU for RESET state until KVM_TDX_INIT_VCPU, as
> +        * userspace needs to define the vCPU model before KVM can initialize
> +        * vCPU state, e.g. to enable x2APIC.
> +        */
> +       WARN_ON_ONCE(init_event);
> +}
> +
>   struct tdx_gmem_post_populate_arg {
>          struct kvm_vcpu *vcpu;
>          __u32 flags;
>