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From: Andrew Jones <drjones@redhat.com>
To: kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org
Cc: maz@kernel.org, mark.rutland@arm.com
Subject: [PATCH v2] arm64: KVM: Only skip MMIO insn once
Date: Thu, 22 Aug 2019 13:03:05 +0200
Message-ID: <20190822110305.18035-1-drjones@redhat.com> (raw)

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")
Signed-off-by: Andrew Jones <drjones@redhat.com>
v2: move mmio_needed use closer to other mmio state use [maz]

 virt/kvm/arm/mmio.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/virt/kvm/arm/mmio.c b/virt/kvm/arm/mmio.c
index a8a6a0c883f1..6af5c91337f2 100644
--- 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. */

             reply index

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-22 11:03 Andrew Jones [this message]
2019-08-22 11:59 ` Mark Rutland
2019-08-22 12:21 ` Marc Zyngier

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