KVM: x86: Introduce MSR filtering

It's not desireable to have all MSRs always handled by KVM kernel space. Some
MSRs would be useful to handle in user space to either emulate behavior (like
uCode updates) or differentiate whether they are valid based on the CPU model.

To allow user space to specify which MSRs it wants to see handled by KVM,
this patch introduces a new ioctl to push filter rules with bitmaps into
KVM. Based on these bitmaps, KVM can then decide whether to reject MSR access.
With the addition of KVM_CAP_X86_USER_SPACE_MSR it can also deflect the
denied MSR events to user space to operate on.

If no filter is populated, MSR handling stays identical to before.

Signed-off-by: Alexander Graf <graf@amazon.com>

Message-Id: <20200925143422.21718-8-graf@amazon.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index f4a2443219bc..dc7a58b39faf 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -87,6 +87,7 @@
 #define KVM_REQ_HV_TLB_FLUSH \
 	KVM_ARCH_REQ_FLAGS(27, KVM_REQUEST_NO_WAKEUP)
 #define KVM_REQ_APF_READY		KVM_ARCH_REQ(28)
+#define KVM_REQ_MSR_FILTER_CHANGED	KVM_ARCH_REQ(29)
 
 #define CR0_RESERVED_BITS                                               \
 	(~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \
@@ -860,6 +861,13 @@ struct kvm_hv {
 	struct kvm_hv_syndbg hv_syndbg;
 };
 
+struct msr_bitmap_range {
+	u32 flags;
+	u32 nmsrs;
+	u32 base;
+	unsigned long *bitmap;
+};
+
 enum kvm_irqchip_mode {
 	KVM_IRQCHIP_NONE,
 	KVM_IRQCHIP_KERNEL,       /* created with KVM_CREATE_IRQCHIP */
@@ -964,6 +972,12 @@ struct kvm_arch {
 	/* Deflect RDMSR and WRMSR to user space when they trigger a #GP */
 	u32 user_space_msr_mask;
 
+	struct {
+		u8 count;
+		bool default_allow:1;
+		struct msr_bitmap_range ranges[16];
+	} msr_filter;
+
 	struct kvm_pmu_event_filter *pmu_event_filter;
 	struct task_struct *nx_lpage_recovery_thread;
 };