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Reply-To: Sean Christopherson <seanjc@google.com>
Date:   Tue, 20 Sep 2022 23:31:24 +0000
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Message-ID: <20220920233134.940511-19-seanjc@google.com>
Subject: [PATCH v3 18/28] KVM: x86: Disable APIC logical map if logical ID
 covers multiple MDAs
From:   Sean Christopherson <seanjc@google.com>
To:     Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Alejandro Jimenez <alejandro.j.jimenez@oracle.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
        Maxim Levitsky <mlevitsk@redhat.com>,
        Li RongQing <lirongqing@baidu.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

Disable the optimized APIC logical map if a logical ID covers multiple
MDAs, i.e. if a vCPU has multiple bits set in its ID.  In logical mode,
events match if "ID & MDA != 0", i.e. creating an entry for only the
first bit can cause interrupts to be missed.

Note, creating an entry for every bit is also wrong as KVM would generate
IPIs for every matching bit.  It would be possible to teach KVM to play
nice with this edge case, but it is very much an edge case and probably
not used in any real world OS, i.e. it's not worth optimizing.

Fixes: 1e08ec4a130e ("KVM: optimize apic interrupt delivery")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 arch/x86/kvm/lapic.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 7a39d7be4cc9..a12360fd4df6 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -338,8 +338,14 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
 			continue;
 		}
 
-		if (mask)
-			cluster[ffs(mask) - 1] = apic;
+		if (!mask)
+			continue;
+
+		if (!is_power_of_2(mask)) {
+			new->logical_mode = KVM_APIC_MODE_MAP_DISABLED;
+			continue;
+		}
+		cluster[ffs(mask) - 1] = apic;
 	}
 out:
 	old = rcu_dereference_protected(kvm->arch.apic_map,
-- 
2.37.3.968.ga6b4b080e4-goog