[PATCH 2/2] KVM: SVM: Add enable_ipiv param, skip physical ID programming if disabled

[Sean Christopherson]

Let userspace "disable" IPI virtualization via an enable_ipiv module param
by programming a dummy entry instead of the vCPU's actual backing entry in
the physical ID table.  SVM doesn't provide a way to actually disable IPI
virtualization in hardware, but by leaving all entries blank, every IPI in
the guest (except for self-IPIs) will generate a VM-Exit.

Providing a way to effectively disable IPI virtualization will allow KVM
to safely enable AVIC on hardware that is suseptible to erratum #1235,
which causes hardware to sometimes fail to detect that the IsRunning bit
has been cleared by software.

All credit goes to Maxim for the idea!

Suggested-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/svm/avic.c | 15 ++++++++++++++-
 arch/x86/kvm/svm/svm.c  |  3 +++
 arch/x86/kvm/svm/svm.h  |  1 +
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index fa87b6853f1d..fc804bb84394 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -310,7 +310,20 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
 		    AVIC_PHYSICAL_ID_ENTRY_VALID_MASK;
 	WRITE_ONCE(table[id], new_entry);
 
-	svm->avic_physical_id_entry = &table[id];
+	/*
+	 * IPI virtualization is bundled with AVIC, but effectively can be
+	 * disabled simply by never marking vCPUs as running in the physical ID
+	 * table.  Use a dummy entry to avoid conditionals in the runtime code,
+	 * and to keep the IOMMU coordination logic as simple as possible.  The
+	 * entry in the table also needs to be valid (see above), otherwise KVM
+	 * will ignore IPIs due to thinking the target doesn't exist.
+	 */
+	if (enable_ipiv) {
+		svm->avic_physical_id_entry = &table[id];
+	} else {
+		svm->ipiv_disabled_backing_entry = table[id];
+		svm->avic_physical_id_entry = &svm->ipiv_disabled_backing_entry;
+	}
 
 	return 0;
 }
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index acdd0b89e471..bc40ffb5c47c 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -227,6 +227,8 @@ module_param(tsc_scaling, int, 0444);
 static bool avic;
 module_param(avic, bool, 0444);
 
+module_param(enable_ipiv, bool, 0444);
+
 bool __read_mostly dump_invalid_vmcb;
 module_param(dump_invalid_vmcb, bool, 0644);
 
@@ -5252,6 +5254,7 @@ static __init int svm_hardware_setup(void)
 	enable_apicv = avic = avic && avic_hardware_setup();
 
 	if (!enable_apicv) {
+		enable_ipiv = false;
 		svm_x86_ops.vcpu_blocking = NULL;
 		svm_x86_ops.vcpu_unblocking = NULL;
 		svm_x86_ops.vcpu_get_apicv_inhibit_reasons = NULL;
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 147516617f88..7a1fc9325d74 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -264,6 +264,7 @@ struct vcpu_svm {
 
 	u32 ldr_reg;
 	u32 dfr_reg;
+	u64 ipiv_disabled_backing_entry;
 	u64 *avic_physical_id_entry;
 
 	/*
-- 
2.42.0.582.g8ccd20d70d-goog


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