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 <[email protected]>
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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