KVM: x86: Mark CR4.TSD as being possibly owned by the guest

Mark CR4.TSD as being possibly owned by the guest as that is indeed the
case on VMX.  Without TSD being tagged as possibly owned by the guest, a
targeted read of CR4 to get TSD could observe a stale value.  This bug
is benign in the current code base as the sole consumer of TSD is the
emulator (for RDTSC) and the emulator always "reads" the entirety of CR4
when grabbing bits.

Add a build-time assertion in to ensure VMX doesn't hand over more CR4
bits without also updating x86.

Fixes: 52ce3c21 ("x86,kvm,vmx: Don't trap writes to CR4.TSD")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200703040422.31536-2-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/kvm_cache_regs.h

@@ -7,7 +7,7 @@
 #define KVM_POSSIBLE_CR0_GUEST_BITS X86_CR0_TS
 #define KVM_POSSIBLE_CR4_GUEST_BITS				  \
 	(X86_CR4_PVI | X86_CR4_DE | X86_CR4_PCE | X86_CR4_OSFXSR  \
-	 | X86_CR4_OSXMMEXCPT | X86_CR4_LA57 | X86_CR4_PGE)
+	 | X86_CR4_OSXMMEXCPT | X86_CR4_LA57 | X86_CR4_PGE | X86_CR4_TSD)
 
 #define BUILD_KVM_GPR_ACCESSORS(lname, uname)				      \
 static __always_inline unsigned long kvm_##lname##_read(struct kvm_vcpu *vcpu)\

arch/x86/kvm/vmx/vmx.c

@@ -4034,6 +4034,8 @@ void vmx_set_constant_host_state(struct vcpu_vmx *vmx)
 
 void set_cr4_guest_host_mask(struct vcpu_vmx *vmx)
 {
+	BUILD_BUG_ON(KVM_CR4_GUEST_OWNED_BITS & ~KVM_POSSIBLE_CR4_GUEST_BITS);
+
 	vmx->vcpu.arch.cr4_guest_owned_bits = KVM_CR4_GUEST_OWNED_BITS;
 	if (enable_ept)
 		vmx->vcpu.arch.cr4_guest_owned_bits |= X86_CR4_PGE;