Re: [PATCH v10 13/28] KVM: VMX: Virtualize FRED nested exception tracking

[Binbin Wu]

On 9/12/2026 5:36 AM, Sohil Mehta wrote:
> From: "Xin Li (Intel)" <xin@xxxxxxxxx>
> 
> Set the VMX nested exception bit in VM-entry interruption information
> field when injecting a nested exception using FRED event delivery to
> ensure:
>   1) A nested exception is injected on a correct stack level.
>   2) The nested bit defined in FRED stack frame is set.
> 
> The event stack level used by FRED event delivery depends on whether
> the event was a nested exception encountered during delivery of an
> earlier event, because a nested exception is "regarded" as happening
> on ring 0.  E.g., when #PF is configured to use stack level 1 in
> IA32_FRED_STKLVLS MSR:
>   - nested #PF will be delivered on the stack pointed by IA32_FRED_RSP1
>     MSR when encountered in ring 3 and ring 0.
>   - normal #PF will be delivered on the stack pointed by IA32_FRED_RSP0
>     MSR when encountered in ring 3.
> 
> The VMX nested-exception support ensures a correct event stack level is
> chosen when a VM entry injects a nested exception.
> 
> In addition, save/restore the nested flag of an exception during VM
> save/restore and live migration to ensure a correct event stack level
> is chosen when a nested exception is injected through FRED event
> delivery across VM save/restore and live migration.
> 
> Note, like the other fields of struct kvm_queued_exception, the nested
> flag is meaningful only while the exception is pending or injected. It
> is not consulted otherwise, and so doesn't need to be cleared when an
> exception is dropped.

Just try to understand the sentence.
The patch clears is_nested alongside pending/injected in two places:
1. kvm_clear_exception_queue()
2. When the exception from userspace is converted to a nested vm exit.

So are they needed?

> 
> Signed-off-by: Xin Li (Intel) <xin@xxxxxxxxx>
> [ Sean: reworked kvm_requeue_exception() to simply the code changes ]
                                                ^
simplify

> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
> Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx>

[...]

> @@ -8978,6 +8993,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
>  					   ex->has_payload, ex->payload);
>  		ex->injected = false;
>  		ex->pending = false;
> +		ex->is_nested = false;
>  	}
>  	vcpu->arch.exception_from_userspace = false;
>  
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 0f5919b092e4..2d6bae66cfae 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -158,6 +158,7 @@ static inline void kvm_clear_exception_queue(struct kvm_vcpu *vcpu)
>  {
>  	vcpu->arch.exception.pending = false;
>  	vcpu->arch.exception.injected = false;
> +	vcpu->arch.exception.is_nested = false;
>  	vcpu->arch.exception_vmexit.pending = false;
>  }