KVM: arm/arm64: Only skip MMIO insn once

If after an MMIO exit to userspace a VCPU is immediately run with an
immediate_exit request, such as when a signal is delivered or an MMIO
emulation completion is needed, then the VCPU completes the MMIO
emulation and immediately returns to userspace. As the exit_reason
does not get changed from KVM_EXIT_MMIO in these cases we have to
be careful not to complete the MMIO emulation again, when the VCPU is
eventually run again, because the emulation does an instruction skip
(and doing too many skips would be a waste of guest code :-) We need
to use additional VCPU state to track if the emulation is complete.
As luck would have it, we already have 'mmio_needed', which even
appears to be used in this way by other architectures already.

Fixes: 0d640732dbeb ("arm64: KVM: Skip MMIO insn after emulation")
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>

diff --git a/virt/kvm/arm/mmio.c b/virt/kvm/arm/mmio.c
index a8a6a0c883f1ba4585610cad9d1e14bc535d4d33..6af5c91337f251fc4a722afb80f9c60d78a0f65e 100644 (file)
--- a/virt/kvm/arm/mmio.c
+++ b/virt/kvm/arm/mmio.c
@@ -86,6 +86,12 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu, struct kvm_run *run)
        unsigned int len;
        int mask;
 
+       /* Detect an already handled MMIO return */
+       if (unlikely(!vcpu->mmio_needed))
+               return 0;
+
+       vcpu->mmio_needed = 0;
+
        if (!run->mmio.is_write) {
                len = run->mmio.len;
                if (len > sizeof(unsigned long))
@@ -188,6 +194,7 @@ int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
        run->mmio.is_write      = is_write;
        run->mmio.phys_addr     = fault_ipa;
        run->mmio.len           = len;
+       vcpu->mmio_needed       = 1;
 
        if (!ret) {
                /* We handled the access successfully in the kernel. */