[PATCH] KVM: x86: Extend KVM_SET_VCPU_EVENTS with selective updates

[Jan Kiszka]

User space may not want to overwrite asynchronously changing VCPU event
states on write-back. So allow to skip nmi.pending and sipi_vector by
setting corresponding bits in the flags field of kvm_vcpu_events.

Signed-off-by: Jan Kiszka <jan.kis...@siemens.com>
---

 Documentation/kvm/api.txt  |   10 +++++++++-
 arch/x86/include/asm/kvm.h |    4 ++++
 arch/x86/kvm/x86.c         |    9 ++++++---
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/Documentation/kvm/api.txt b/Documentation/kvm/api.txt
index e1a1141..ee07e3a 100644
--- a/Documentation/kvm/api.txt
+++ b/Documentation/kvm/api.txt
@@ -685,7 +685,7 @@ struct kvm_vcpu_events {
                __u8 pad;
        } nmi;
        __u32 sipi_vector;
-       __u32 flags;   /* must be zero */
+       __u32 flags;
 };
 
 4.30 KVM_SET_VCPU_EVENTS
@@ -701,6 +701,14 @@ vcpu.
 
 See KVM_GET_VCPU_EVENTS for the data structure.
 
+Fields that may be modified asynchronously by running VCPUs can be excluded
+from the update. These fields are nmi.pending and sipi_vector. Set the
+corresponding mask bits in the flags field to suppress overwriting their
+current state:
+
+KVM_VCPUEVENT_MASK_NMI_PENDING - do not update nmi.pending
+KVM_VCPUEVENT_MASK_SIPI_VECTOR - do not update sipi_vector
+
 
 5. The kvm_run structure
 
diff --git a/arch/x86/include/asm/kvm.h b/arch/x86/include/asm/kvm.h
index 950df43..acf8585 100644
--- a/arch/x86/include/asm/kvm.h
+++ b/arch/x86/include/asm/kvm.h
@@ -254,6 +254,10 @@ struct kvm_reinject_control {
        __u8 reserved[31];
 };
 
+/* When set in flags, skip corresponding fields on KVM_SET_VCPU_EVENTS */
+#define KVM_VCPUEVENT_MASK_NMI_PENDING 0x00000001
+#define KVM_VCPUEVENT_MASK_SIPI_VECTOR 0x00000002
+
 /* for KVM_GET/SET_VCPU_EVENTS */
 struct kvm_vcpu_events {
        struct {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index dd15d7a..368843c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1953,7 +1953,8 @@ static void kvm_vcpu_ioctl_x86_get_vcpu_events(struct 
kvm_vcpu *vcpu,
 static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
                                              struct kvm_vcpu_events *events)
 {
-       if (events->flags)
+       if (events->flags &
+           ~(KVM_VCPUEVENT_MASK_NMI_PENDING | KVM_VCPUEVENT_MASK_SIPI_VECTOR))
                return -EINVAL;
 
        vcpu_load(vcpu);
@@ -1970,10 +1971,12 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct 
kvm_vcpu *vcpu,
                kvm_pic_clear_isr_ack(vcpu->kvm);
 
        vcpu->arch.nmi_injected = events->nmi.injected;
-       vcpu->arch.nmi_pending = events->nmi.pending;
+       if (!(events->flags & KVM_VCPUEVENT_MASK_NMI_PENDING))
+               vcpu->arch.nmi_pending = events->nmi.pending;
        kvm_x86_ops->set_nmi_mask(vcpu, events->nmi.masked);
 
-       vcpu->arch.sipi_vector = events->sipi_vector;
+       if (!(events->flags & KVM_VCPUEVENT_MASK_SIPI_VECTOR))
+               vcpu->arch.sipi_vector = events->sipi_vector;
 
        vcpu_put(vcpu);
 

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