linux-kernel - [PATCH 1/3] KVM: x86: VMX: __kvm_apic_update

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Message-Id: <20230718091310.119672-2-mlevitsk@redhat.com>
Date:   Tue, 18 Jul 2023 12:13:08 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     kvm@...r.kernel.org
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH 1/3] KVM: x86: VMX: __kvm_apic_update_irr must update the IRR atomically

If APICv is inhibited, then IPIs from peer vCPUs are done by
atomically setting bits in IRR.

This means, that when __kvm_apic_update_irr copies PIR to IRR,
it has to modify IRR atomically as well.

Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
---
 arch/x86/kvm/lapic.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index e542cf285b5184..b3f57e0f0d64ae 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -627,15 +627,19 @@ bool __kvm_apic_update_irr(u32 *pir, void *regs, int *max_irr)
 
 	for (i = vec = 0; i <= 7; i++, vec += 32) {
 		pir_val = READ_ONCE(pir[i]);
-		irr_val = *((u32 *)(regs + APIC_IRR + i * 0x10));
+		irr_val = READ_ONCE(*((u32 *)(regs + APIC_IRR + i * 0x10)));
+
 		if (pir_val) {
+			pir_val = xchg(&pir[i], 0);
+
+			while (!try_cmpxchg(((u32 *)(regs + APIC_IRR + i * 0x10)),
+			       &irr_val, irr_val | pir_val));
+
 			prev_irr_val = irr_val;
-			irr_val |= xchg(&pir[i], 0);
-			*((u32 *)(regs + APIC_IRR + i * 0x10)) = irr_val;
-			if (prev_irr_val != irr_val) {
-				max_updated_irr =
-					__fls(irr_val ^ prev_irr_val) + vec;
-			}
+			irr_val |= pir_val;
+
+			if (prev_irr_val != irr_val)
+				max_updated_irr = __fls(irr_val ^ prev_irr_val) + vec;
 		}
 		if (irr_val)
 			*max_irr = __fls(irr_val) + vec;
-- 
2.26.3