KVM: x86: Drop superfluous caching of KVM_ASYNC_PF_SEND_ALWAYS

Drop kvm_vcpu_arch.apf.send_always and instead use msr_en_val as the source
of truth to reduce the probability of operating on stale data.  This fixes
flaws where KVM fails to update send_always when APF is explicitly
disabled by the guest or implicitly disabled by KVM on INIT.  Absent other
bugs, the flaws are benign as KVM *shouldn't* consume send_always when PV
APF support is disabled.

Simply delete the field, as there's zero benefit to maintaining a separate
"cache" of the state.

Opportunistically turn the enabled vs. disabled logic at the end of
kvm_pv_enable_async_pf() into an if-else instead of using an early return,
e.g. so that it's more obvious that both paths are "success" paths.

Fixes: 6adba5274206 ("KVM: Let host know whether the guest can handle async PF in non-userspace context.")
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://patch.msgid.link/20260406225359.1245490-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

2 files changed, 4 insertions, 9 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 5644dc9f08a4..2b986a733cd6 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1057,7 +1057,6 @@ struct kvm_vcpu_arch {
 		u16 vec;
 		u32 id;
 		u32 host_apf_flags;
-		bool send_always;
 		bool pageready_pending;
 	} apf;
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4bffcea3ede9..b01f9a4d3363 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3660,16 +3660,12 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
 
 	vcpu->arch.apf.msr_en_val = data;
 
-	if (!__kvm_pv_async_pf_enabled(data)) {
+	if (__kvm_pv_async_pf_enabled(data)) {
+		kvm_async_pf_wakeup_all(vcpu);
+	} else {
 		kvm_clear_async_pf_completion_queue(vcpu);
 		kvm_async_pf_hash_reset(vcpu);
-		return 0;
 	}
-
-	vcpu->arch.apf.send_always = (data & KVM_ASYNC_PF_SEND_ALWAYS);
-
-	kvm_async_pf_wakeup_all(vcpu);
-
 	return 0;
 }
 
@@ -14008,7 +14004,7 @@ static bool kvm_can_deliver_async_pf(struct kvm_vcpu *vcpu)
 	if (!kvm_pv_async_pf_enabled(vcpu))
 		return false;
 
-	if (!vcpu->arch.apf.send_always &&
+	if (!(vcpu->arch.apf.msr_en_val & KVM_ASYNC_PF_SEND_ALWAYS) &&
 	    (vcpu->arch.guest_state_protected || !kvm_x86_call(get_cpl)(vcpu)))
 		return false;