KVM: x86: Save and reload SSP to/from SMRAM

Save CET SSP to SMRAM on SMI and reload it on RSM. KVM emulates HW arch
behavior when guest enters/leaves SMM mode,i.e., save registers to SMRAM
at the entry of SMM and reload them at the exit to SMM. Per SDM, SSP is
one of such registers on 64-bit Arch, and add the support for SSP.

Suggested-by: Sean Christopherson <[e-mail]>
Signed-off-by: Yang Weijiang <[e-mail]>
Tested-by: Mathias Krause <[e-mail]>
Tested-by: John Allen <[e-mail]>
Tested-by: Rick Edgecombe <[e-mail]>
Signed-off-by: Chao Gao <[e-mail]>
Reviewed-by: Binbin Wu <[e-mail]>
Reviewed-by: Xiaoyao Li <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

arch/x86/kvm/smm.c

···
 	enter_smm_save_seg_64(vcpu, &smram->gs, VCPU_SREG_GS);
 
 	smram->int_shadow = kvm_x86_call(get_interrupt_shadow)(vcpu);
+
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) &&
+	    kvm_msr_read(vcpu, MSR_KVM_INTERNAL_GUEST_SSP, &smram->ssp))
+		kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
 }
 #endif
 
···
 
 	kvm_x86_call(set_interrupt_shadow)(vcpu, 0);
 	ctxt->interruptibility = (u8)smstate->int_shadow;
+
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) &&
+	    kvm_msr_write(vcpu, MSR_KVM_INTERNAL_GUEST_SSP, smstate->ssp))
+		return X86EMUL_UNHANDLEABLE;
 
 	return X86EMUL_CONTINUE;
 }

arch/x86/kvm/smm.h

···
 	u32 smbase;
 	u32 reserved4[5];
 
-	/* ssp and svm_* fields below are not implemented by KVM */
 	u64 ssp;
+	/* svm_* fields below are not implemented by KVM */
 	u64 svm_guest_pat;
 	u64 svm_host_efer;
 	u64 svm_host_cr4;