linux-kernel - [PATCH] kvm: lapic: fix broken vcpu hotplug

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Date:   Mon, 22 Jun 2020 12:08:30 -0400
From:   Igor Mammedov <imammedo@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     pbonzini@...hat.com, sean.j.christopherson@...el.com,
        vkuznets@...hat.com, kvm@...r.kernel.org, wanpengli@...cent.com
Subject: [PATCH] kvm: lapic: fix broken vcpu hotplug

Guest fails to online hotplugged CPU with error
  smpboot: do_boot_cpu failed(-1) to wakeup CPU#4

It's caused by the fact that kvm_apic_set_state(), which used to call
recalculate_apic_map() unconditionally and pulled hotplugged CPU into
apic map, is updating map conditionally [1] on state change which doesn't
happen in this case and apic map update is skipped.

Note:
new CPU during kvm_arch_vcpu_create() is not visible to
kvm_recalculate_apic_map(), so all related update calls endup
as NOP and only follow up kvm_apic_set_state() used to trigger map
update that counted in hotplugged CPU.
Fix issue by forcing unconditional update from kvm_apic_set_state(),
like it used to be.

1)
Fixes: (4abaffce4d25a KVM: LAPIC: Recalculate apic map in batch)
Signed-off-by: Igor Mammedov <imammedo@...hat.com>
---
PS:
it's alternative to full revert of [1], I've posted earlier
https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2205600.html
so fii free to pick up whatever is better by now
---
 arch/x86/kvm/lapic.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 34a7e0533dad..5696831d4005 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2556,6 +2556,7 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
 	struct kvm_lapic *apic = vcpu->arch.apic;
 	int r;

+	apic->vcpu->kvm->arch.apic_map_dirty = true;
 	kvm_lapic_set_base(vcpu, vcpu->arch.apic_base);
 	/* set SPIV separately to get count of SW disabled APICs right */
 	apic_set_spiv(apic, *((u32 *)(s->regs + APIC_SPIV)));
-- 
2.26.2