x86/sev: Move MSR-based VMGEXITs for CPUID to helper

This code will also be used later for SEV-SNP-validated CPUID code in
some cases, so move it to a common helper.

While here, also add a check to terminate in cases where the CPUID
function/subfunction is indexed and the subfunction is non-zero, since
the GHCB MSR protocol does not support non-zero subfunctions.
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220307213356.2797205-32-brijesh.singh@amd.com

arch/x86/boot/compressed/sev.c

@@ -20,6 +20,7 @@
 #include <asm/fpu/xcr.h>
 #include <asm/ptrace.h>
 #include <asm/svm.h>
+#include <asm/cpuid.h>
 
 #include "error.h"
 #include "../msr.h"

arch/x86/kernel/sev-shared.c

@@ -14,6 +14,16 @@
 #define has_cpuflag(f)	boot_cpu_has(f)
 #endif
 
+/* I/O parameters for CPUID-related helpers */
+struct cpuid_leaf {
+	u32 fn;
+	u32 subfn;
+	u32 eax;
+	u32 ebx;
+	u32 ecx;
+	u32 edx;
+};
+
 /*
  * Since feature negotiation related variables are set early in the boot
  * process they must reside in the .data section so as not to be zeroed
@@ -194,6 +204,44 @@ enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb, bool set_ghcb_msr,
 	return verify_exception_info(ghcb, ctxt);
 }
 
+static int __sev_cpuid_hv(u32 fn, int reg_idx, u32 *reg)
+{
+	u64 val;
+
+	sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(fn, reg_idx));
+	VMGEXIT();
+	val = sev_es_rd_ghcb_msr();
+	if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
+		return -EIO;
+
+	*reg = (val >> 32);
+
+	return 0;
+}
+
+static int sev_cpuid_hv(struct cpuid_leaf *leaf)
+{
+	int ret;
+
+	/*
+	 * MSR protocol does not support fetching non-zero subfunctions, but is
+	 * sufficient to handle current early-boot cases. Should that change,
+	 * make sure to report an error rather than ignoring the index and
+	 * grabbing random values. If this issue arises in the future, handling
+	 * can be added here to use GHCB-page protocol for cases that occur late
+	 * enough in boot that GHCB page is available.
+	 */
+	if (cpuid_function_is_indexed(leaf->fn) && leaf->subfn)
+		return -EINVAL;
+
+	ret =         __sev_cpuid_hv(leaf->fn, GHCB_CPUID_REQ_EAX, &leaf->eax);
+	ret = ret ? : __sev_cpuid_hv(leaf->fn, GHCB_CPUID_REQ_EBX, &leaf->ebx);
+	ret = ret ? : __sev_cpuid_hv(leaf->fn, GHCB_CPUID_REQ_ECX, &leaf->ecx);
+	ret = ret ? : __sev_cpuid_hv(leaf->fn, GHCB_CPUID_REQ_EDX, &leaf->edx);
+
+	return ret;
+}
+
 /*
  * Boot VC Handler - This is the first VC handler during boot, there is no GHCB
  * page yet, so it only supports the MSR based communication with the
@@ -201,40 +249,23 @@ enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb, bool set_ghcb_msr,
  */
 void __init do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)
 {
+	unsigned int subfn = lower_bits(regs->cx, 32);
 	unsigned int fn = lower_bits(regs->ax, 32);
-	unsigned long val;
+	struct cpuid_leaf leaf;
 
 	/* Only CPUID is supported via MSR protocol */
 	if (exit_code != SVM_EXIT_CPUID)
 		goto fail;
 
-	sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(fn, GHCB_CPUID_REQ_EAX));
-	VMGEXIT();
-	val = sev_es_rd_ghcb_msr();
-	if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
-		goto fail;
-	regs->ax = val >> 32;
-
-	sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(fn, GHCB_CPUID_REQ_EBX));
-	VMGEXIT();
-	val = sev_es_rd_ghcb_msr();
-	if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
-		goto fail;
-	regs->bx = val >> 32;
-
-	sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(fn, GHCB_CPUID_REQ_ECX));
-	VMGEXIT();
-	val = sev_es_rd_ghcb_msr();
-	if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
+	leaf.fn = fn;
+	leaf.subfn = subfn;
+	if (sev_cpuid_hv(&leaf))
 		goto fail;
-	regs->cx = val >> 32;
 
-	sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(fn, GHCB_CPUID_REQ_EDX));
-	VMGEXIT();
-	val = sev_es_rd_ghcb_msr();
-	if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
-		goto fail;
-	regs->dx = val >> 32;
+	regs->ax = leaf.eax;
+	regs->bx = leaf.ebx;
+	regs->cx = leaf.ecx;
+	regs->dx = leaf.edx;
 
 	/*
 	 * This is a VC handler and the #VC is only raised when SEV-ES is

arch/x86/kernel/sev.c

@@ -33,6 +33,7 @@
 #include <asm/smp.h>
 #include <asm/cpu.h>
 #include <asm/apic.h>
+#include <asm/cpuid.h>
 
 #define DR7_RESET_VALUE        0x400