KVM: x86: Add kvm_x86_ops hook to short circuit emulation

Replace the existing kvm_x86_ops.need_emulation_on_page_fault() with a
more generic is_emulatable(), and unconditionally call the new function
in x86_emulate_instruction().

KVM will use the generic hook to support multiple security related
technologies that prevent emulation in one way or another.  Similar to
the existing AMD #NPF case where emulation of the current instruction is
not possible due to lack of information, AMD's SEV-ES and Intel's SGX
and TDX will introduce scenarios where emulation is impossible due to
the guest's register state being inaccessible.  And again similar to the
existing #NPF case, emulation can be initiated by kvm_mmu_page_fault(),
i.e. outside of the control of vendor-specific code.

While the cause and architecturally visible behavior of the various
cases are different, e.g. SGX will inject a #UD, AMD #NPF is a clean
resume or complete shutdown, and SEV-ES and TDX "return" an error, the
impact on the common emulation code is identical: KVM must stop
emulation immediately and resume the guest.

Query is_emulatable() in handle_ud() as well so that the
force_emulation_prefix code doesn't incorrectly modify RIP before
calling emulate_instruction() in the absurdly unlikely scenario that
KVM encounters forced emulation in conjunction with "do not emulate".

Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200915232702.15945-1-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -1221,7 +1221,7 @@ struct kvm_x86_ops {
 
 	int (*get_msr_feature)(struct kvm_msr_entry *entry);
 
-	bool (*need_emulation_on_page_fault)(struct kvm_vcpu *vcpu);
+	bool (*can_emulate_instruction)(struct kvm_vcpu *vcpu, void *insn, int insn_len);
 
 	bool (*apic_init_signal_blocked)(struct kvm_vcpu *vcpu);
 	int (*enable_direct_tlbflush)(struct kvm_vcpu *vcpu);

arch/x86/kvm/mmu/mmu.c

@@ -5485,18 +5485,6 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 error_code,
 	if (!mmio_info_in_cache(vcpu, cr2_or_gpa, direct) && !is_guest_mode(vcpu))
 		emulation_type |= EMULTYPE_ALLOW_RETRY_PF;
 emulate:
-	/*
-	 * On AMD platforms, under certain conditions insn_len may be zero on #NPF.
-	 * This can happen if a guest gets a page-fault on data access but the HW
-	 * table walker is not able to read the instruction page (e.g instruction
-	 * page is not present in memory). In those cases we simply restart the
-	 * guest, with the exception of AMD Erratum 1096 which is unrecoverable.
-	 */
-	if (unlikely(insn && !insn_len)) {
-		if (!kvm_x86_ops.need_emulation_on_page_fault(vcpu))
-			return 1;
-	}
-
 	return x86_emulate_instruction(vcpu, cr2_or_gpa, emulation_type, insn,
 				       insn_len);
 }

arch/x86/kvm/svm/svm.c

@@ -3984,19 +3984,10 @@ static void enable_smi_window(struct kvm_vcpu *vcpu)
 	}
 }
 
-static bool svm_need_emulation_on_page_fault(struct kvm_vcpu *vcpu)
+static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, void *insn, int insn_len)
 {
-	unsigned long cr4 = kvm_read_cr4(vcpu);
-	bool smep = cr4 & X86_CR4_SMEP;
-	bool smap = cr4 & X86_CR4_SMAP;
-	bool is_user = svm_get_cpl(vcpu) == 3;
-
-	/*
-	 * If RIP is invalid, go ahead with emulation which will cause an
-	 * internal error exit.
-	 */
-	if (!kvm_vcpu_gfn_to_memslot(vcpu, kvm_rip_read(vcpu) >> PAGE_SHIFT))
-		return true;
+	bool smep, smap, is_user;
+	unsigned long cr4;
 
 	/*
 	 * Detect and workaround Errata 1096 Fam_17h_00_0Fh.
@@ -4038,6 +4029,20 @@ static bool svm_need_emulation_on_page_fault(struct kvm_vcpu *vcpu)
 	 * instruction pointer so we will not able to workaround it. Lets
 	 * print the error and request to kill the guest.
 	 */
+	if (likely(!insn || insn_len))
+		return true;
+
+	/*
+	 * If RIP is invalid, go ahead with emulation which will cause an
+	 * internal error exit.
+	 */
+	if (!kvm_vcpu_gfn_to_memslot(vcpu, kvm_rip_read(vcpu) >> PAGE_SHIFT))
+		return true;
+
+	cr4 = kvm_read_cr4(vcpu);
+	smep = cr4 & X86_CR4_SMEP;
+	smap = cr4 & X86_CR4_SMAP;
+	is_user = svm_get_cpl(vcpu) == 3;
 	if (smap && (!smep || is_user)) {
 		if (!sev_guest(vcpu->kvm))
 			return true;
@@ -4199,7 +4204,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
 	.mem_enc_reg_region = svm_register_enc_region,
 	.mem_enc_unreg_region = svm_unregister_enc_region,
 
-	.need_emulation_on_page_fault = svm_need_emulation_on_page_fault,
+	.can_emulate_instruction = svm_can_emulate_instruction,
 
 	.apic_init_signal_blocked = svm_apic_init_signal_blocked,
 };

arch/x86/kvm/vmx/vmx.c

@@ -1561,6 +1561,11 @@ static int vmx_rtit_ctl_check(struct kvm_vcpu *vcpu, u64 data)
 	return 0;
 }
 
+static bool vmx_can_emulate_instruction(struct kvm_vcpu *vcpu, void *insn, int insn_len)
+{
+	return true;
+}
+
 static int skip_emulated_instruction(struct kvm_vcpu *vcpu)
 {
 	unsigned long rip, orig_rip;
@@ -7749,11 +7754,6 @@ static void enable_smi_window(struct kvm_vcpu *vcpu)
 	/* RSM will cause a vmexit anyway.  */
 }
 
-static bool vmx_need_emulation_on_page_fault(struct kvm_vcpu *vcpu)
-{
-	return false;
-}
-
 static bool vmx_apic_init_signal_blocked(struct kvm_vcpu *vcpu)
 {
 	return to_vmx(vcpu)->nested.vmxon;
@@ -7908,7 +7908,7 @@ static struct kvm_x86_ops vmx_x86_ops __initdata = {
 	.pre_leave_smm = vmx_pre_leave_smm,
 	.enable_smi_window = enable_smi_window,
 
-	.need_emulation_on_page_fault = vmx_need_emulation_on_page_fault,
+	.can_emulate_instruction = vmx_can_emulate_instruction,
 	.apic_init_signal_blocked = vmx_apic_init_signal_blocked,
 	.migrate_timers = vmx_migrate_timers,
 };

arch/x86/kvm/x86.c

@@ -3222,8 +3222,9 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		 * even when not intercepted. AMD manual doesn't explicitly
 		 * state this but appears to behave the same.
 		 *
-		 * However when userspace wants to read this MSR, we should
-		 * return it's real L1 value so that its restore will be correct.
+		 * Unconditionally return L1's TSC offset on userspace reads
+		 * so that userspace reads and writes always operate on L1's
+		 * offset, e.g. to ensure deterministic behavior for migration.
 		 */
 		u64 tsc_offset = msr_info->host_initiated ? vcpu->arch.l1_tsc_offset :
 							    vcpu->arch.tsc_offset;
@@ -5714,6 +5715,9 @@ int handle_ud(struct kvm_vcpu *vcpu)
 	char sig[5]; /* ud2; .ascii "kvm" */
 	struct x86_exception e;
 
+	if (unlikely(!kvm_x86_ops.can_emulate_instruction(vcpu, NULL, 0)))
+		return 1;
+
 	if (force_emulation_prefix &&
 	    kvm_read_guest_virt(vcpu, kvm_get_linear_rip(vcpu),
 				sig, sizeof(sig), &e) == 0 &&
@@ -6919,7 +6923,10 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 	int r;
 	struct x86_emulate_ctxt *ctxt = vcpu->arch.emulate_ctxt;
 	bool writeback = true;
-	bool write_fault_to_spt = vcpu->arch.write_fault_to_shadow_pgtable;
+	bool write_fault_to_spt;
+
+	if (unlikely(!kvm_x86_ops.can_emulate_instruction(vcpu, insn, insn_len)))
+		return 1;
 
 	vcpu->arch.l1tf_flush_l1d = true;
 
@@ -6927,6 +6934,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 	 * Clear write_fault_to_shadow_pgtable here to ensure it is
 	 * never reused.
 	 */
+	write_fault_to_spt = vcpu->arch.write_fault_to_shadow_pgtable;
 	vcpu->arch.write_fault_to_shadow_pgtable = false;
 	kvm_clear_exception_queue(vcpu);