x86/sev-es: Handle RDTSC(P) Events

Implement a handler for #VC exceptions caused by RDTSC and RDTSCP
instructions. Also make it available in the pre-decompression stage
because the KASLR code uses RDTSC/RDTSCP to gather entropy and some
hypervisors intercept these instructions.
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
[ jroedel@suse.de: - Adapt to #VC handling infrastructure
                   - Make it available early ]
Co-developed-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200907131613.12703-55-joro@8bytes.org

arch/x86/boot/compressed/sev-es.c

@@ -181,6 +181,10 @@ void do_boot_stage2_vc(struct pt_regs *regs, unsigned long exit_code)
 		goto finish;
 
 	switch (exit_code) {
+	case SVM_EXIT_RDTSC:
+	case SVM_EXIT_RDTSCP:
+		result = vc_handle_rdtsc(boot_ghcb, &ctxt, exit_code);
+		break;
 	case SVM_EXIT_IOIO:
 		result = vc_handle_ioio(boot_ghcb, &ctxt);
 		break;

arch/x86/kernel/sev-es-shared.c

@@ -467,3 +467,26 @@ static enum es_result vc_handle_cpuid(struct ghcb *ghcb,
 
 	return ES_OK;
 }
+
+static enum es_result vc_handle_rdtsc(struct ghcb *ghcb,
+				      struct es_em_ctxt *ctxt,
+				      unsigned long exit_code)
+{
+	bool rdtscp = (exit_code == SVM_EXIT_RDTSCP);
+	enum es_result ret;
+
+	ret = sev_es_ghcb_hv_call(ghcb, ctxt, exit_code, 0, 0);
+	if (ret != ES_OK)
+		return ret;
+
+	if (!(ghcb_rax_is_valid(ghcb) && ghcb_rdx_is_valid(ghcb) &&
+	     (!rdtscp || ghcb_rcx_is_valid(ghcb))))
+		return ES_VMM_ERROR;
+
+	ctxt->regs->ax = ghcb->save.rax;
+	ctxt->regs->dx = ghcb->save.rdx;
+	if (rdtscp)
+		ctxt->regs->cx = ghcb->save.rcx;
+
+	return ES_OK;
+}

arch/x86/kernel/sev-es.c

@@ -866,6 +866,10 @@ static enum es_result vc_handle_exitcode(struct es_em_ctxt *ctxt,
 	case SVM_EXIT_WRITE_DR7:
 		result = vc_handle_dr7_write(ghcb, ctxt);
 		break;
+	case SVM_EXIT_RDTSC:
+	case SVM_EXIT_RDTSCP:
+		result = vc_handle_rdtsc(ghcb, ctxt, exit_code);
+		break;
 	case SVM_EXIT_CPUID:
 		result = vc_handle_cpuid(ghcb, ctxt);
 		break;