linux-kernel - [PATCH 12/19] KVM: x86: Disable APIC logical map if logical ID covers multiple MDAs

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Message-ID: <20220831003506.4117148-13-seanjc@google.com>
Date:   Wed, 31 Aug 2022 00:34:59 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Li RongQing <lirongqing@...du.com>
Subject: [PATCH 12/19] KVM: x86: Disable APIC logical map if logical ID covers
 multiple MDAs

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.

Use an impossible value for the "mode" to effectively designate that it's
disabled.  Don't bother adding a dedicated "invalid" value, the mode
handling will be cleaned up in the future and it would take just as much
effort to explain what value is "safe" at this time.

Fixes: 1e08ec4a130e ("KVM: optimize apic interrupt delivery")
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/lapic.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 9dda989a1cf0..82278acae95b 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -300,8 +300,15 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
 		if (!kvm_apic_map_get_logical_dest(new, ldr, &cluster, &mask))
 			continue;

-		if (mask)
-			cluster[ffs(mask) - 1] = apic;
+		if (!mask)
+			continue;
+
+		if (!is_power_of_2(mask)) {
+			new->mode = KVM_APIC_MODE_XAPIC_FLAT |
+				    KVM_APIC_MODE_XAPIC_CLUSTER;
+			continue;
+		}
+		cluster[ffs(mask) - 1] = apic;
 	}
 out:
 	old = rcu_dereference_protected(kvm->arch.apic_map,
-- 
2.37.2.672.g94769d06f0-goog