linux-kernel - [PATCH 2/5] KVM: Fix race between mmio reads and injected interrupts

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Date:	Thu, 11 Jan 2007 10:04:30 -0000
From:	Avi Kivity <avi@...ranet.com>
To:	kvm-devel@...ts.sourceforge.net
Cc:	linux-kernel@...r.kernel.org, akpm@...l.org, mingo@...e.hu
Subject: [PATCH 2/5] KVM: Fix race between mmio reads and injected interrupts

The kvm mmio read path looks like:

 1. guest read faults
 2. kvm emulates read, calls emulator_read_emulated()
 3. fails as a read requires userspace help
 4. exit to userspace
 5. userspace emulates read, kvm sets vcpu->mmio_read_completed
 6. re-enter guest, fault again
 7. kvm emulates read, calls emulator_read_emulated()
 8. succeeds as vcpu->mmio_read_emulated is set
 9. instruction completes and guest is resumed

A problem surfaces if the userspace exit (step 5) also requests an interrupt
injection.  In that case, the guest does not re-execute the original
instruction, but the interrupt handler.  The next time an mmio read is
exectued (likely for a different address), step 3 will find
vcpu->mmio_read_completed set and return the value read for the original
instruction.

The problem manifested itself in a few annoying ways:
- little squares appear randomly on console when switching virtual terminals
- ne2000 fails under nfs read load
- rtl8139 complains about "pci errors" even though the device model is
  incapable of issuing them.

Fix by skipping interrupt injection if an mmio read is pending.

A better fix is to avoid re-entry into the guest, and re-emulating immediately
instead.  However that's a bit more complex.

Signed-off-by: Avi Kivity <avi@...ranet.com>

Index: linux-2.6/drivers/kvm/svm.c
===================================================================
--- linux-2.6.orig/drivers/kvm/svm.c
+++ linux-2.6/drivers/kvm/svm.c
@@ -1407,7 +1407,8 @@ static int svm_vcpu_run(struct kvm_vcpu 
 	int r;

 again:
-	do_interrupt_requests(vcpu, kvm_run);
+	if (!vcpu->mmio_read_completed)
+		do_interrupt_requests(vcpu, kvm_run);

 	clgi();

Index: linux-2.6/drivers/kvm/vmx.c
===================================================================
--- linux-2.6.orig/drivers/kvm/vmx.c
+++ linux-2.6/drivers/kvm/vmx.c
@@ -1717,7 +1717,8 @@ again:
 	vmcs_writel(HOST_GS_BASE, segment_base(gs_sel));
 #endif

-	do_interrupt_requests(vcpu, kvm_run);
+	if (!vcpu->mmio_read_completed)
+		do_interrupt_requests(vcpu, kvm_run);

 	if (vcpu->guest_debug.enabled)
 		kvm_guest_debug_pre(vcpu);
-
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