linux-kernel - [PATCH 3/3] KVM: arm/arm64: vgic: Don't rely on the wrong pending table

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Date:   Tue, 29 Oct 2019 15:19:19 +0800
From:   Zenghui Yu <yuzenghui@...wei.com>
To:     <maz@...nel.org>, <eric.auger@...hat.com>, <james.morse@....com>,
        <julien.thierry.kdev@...il.com>, <suzuki.poulose@....com>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        <kvmarm@...ts.cs.columbia.edu>, <linux-kernel@...r.kernel.org>,
        <wanghaibin.wang@...wei.com>, Zenghui Yu <yuzenghui@...wei.com>
Subject: [PATCH 3/3] KVM: arm/arm64: vgic: Don't rely on the wrong pending table

It's possible that two LPIs locate in the same "byte_offset" but target
two different vcpus, where their pending status are indicated by two
different pending tables.  In such a scenario, using last_byte_offset
optimization will lead KVM relying on the wrong pending table entry.
Let us use last_ptr instead, which can be treated as a byte index into
a pending table and also, can be vcpu specific.

Signed-off-by: Zenghui Yu <yuzenghui@...wei.com>
---

If this patch has done the right thing, we can even add the:

Fixes: 280771252c1b ("KVM: arm64: vgic-v3: KVM_DEV_ARM_VGIC_SAVE_PENDING_TABLES")

But to be honest, I'm not clear about what has this patch actually fixed.
Pending tables should contain all zeros before we flush vgic_irq's pending
status into guest's RAM (thinking that guest should never write anything
into it). So the pending table entry we've read from the guest memory
seems always be zero. And we will always do the right thing even if we
rely on the wrong pending table entry.

I think I must have some misunderstanding here... Please fix me.

 virt/kvm/arm/vgic/vgic-v3.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/virt/kvm/arm/vgic/vgic-v3.c b/virt/kvm/arm/vgic/vgic-v3.c
index 5ef93e5041e1..7cd2e2f81513 100644
--- a/virt/kvm/arm/vgic/vgic-v3.c
+++ b/virt/kvm/arm/vgic/vgic-v3.c
@@ -363,8 +363,8 @@ int vgic_v3_lpi_sync_pending_status(struct kvm *kvm, struct vgic_irq *irq)
 int vgic_v3_save_pending_tables(struct kvm *kvm)
 {
 	struct vgic_dist *dist = &kvm->arch.vgic;
-	int last_byte_offset = -1;
 	struct vgic_irq *irq;
+	gpa_t last_ptr = -1;
 	int ret;
 	u8 val;

@@ -384,11 +384,11 @@ int vgic_v3_save_pending_tables(struct kvm *kvm)
 		bit_nr = irq->intid % BITS_PER_BYTE;
 		ptr = pendbase + byte_offset;

-		if (byte_offset != last_byte_offset) {
+		if (ptr != last_ptr) {
 			ret = kvm_read_guest_lock(kvm, ptr, &val, 1);
 			if (ret)
 				return ret;
-			last_byte_offset = byte_offset;
+			last_ptr = ptr;
 		}

 		stored = val & (1U << bit_nr);
-- 
2.19.1