Re: [PATCH] KVM: x86: Drop redundant call to kvm_deliver_exception_payload()

[Sean Christopherson <seanjc@google.com>]

On Mon, Mar 02, 2026, Yosry Ahmed wrote:
> In kvm_check_and_inject_events(), kvm_deliver_exception_payload() is
> called for pending #DB exceptions. However, shortly after, the
> per-vendor inject_exception callbacks are made. Both
> vmx_inject_exception() and svm_inject_exception() unconditionally call
> kvm_deliver_exception_payload(), so the call in
> kvm_check_and_inject_events() is redundant.
> 
> Note that the extra call for pending #DB exceptions is harmless, as
> kvm_deliver_exception_payload() clears exception.has_payload after the
> first call.
> 
> The call in kvm_check_and_inject_events() was added in commit
> f10c729ff965 ("kvm: vmx: Defer setting of DR6 until #DB delivery"). At
> that point, the call was likely needed because svm_queue_exception()
> checked whether an exception for L2 is intercepted by L1 before calling
> kvm_deliver_exception_payload(), as SVM did not have a
> check_nested_events callback. Since DR6 is updated before the #DB
> intercept in SVM (unlike VMX), it was necessary to deliver the DR6
> payload before calling svm_queue_exception().
> 
> After that, commit 7c86663b68ba ("KVM: nSVM: inject exceptions via
> svm_check_nested_events") added a check_nested_events callback for SVM,
> which checked for L1 intercepts for L2's exceptions, and delivered the
> the payload appropriately before the intercept. At that point,
> svm_queue_exception() started calling kvm_deliver_exception_payload()
> unconditionally, and the call to kvm_deliver_exception_payload() from
> its caller became redundant.

Nice!  I vaguely remember staring at this code when working on 5623f751bd9c
("KVM: x86: Treat #DBs from the emulator as fault-like (code and DR7.GD=1)"),
but never pieced together that it was redundant.