[PATCH RFC] kvm: x86: Expose AVX512_BF16 feature to guest

[Jing Liu]

AVX512 BFLOAT16 instructions support 16-bit BFLOAT16 floating-point
format (BF16) for deep learning optimization.

Intel adds AVX512 BFLOAT16 feature in CooperLake, which is CPUID.7.1.EAX[5].

Detailed information of the CPUID bit can be found here,
https://software.intel.com/sites/default/files/managed/c5/15/\
architecture-instruction-set-extensions-programming-reference.pdf.

Signed-off-by: Jing Liu <jing2.liu@xxxxxxxxxxxxxxx>
---
 arch/x86/kvm/cpuid.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index e18a9f9..10be53f 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -484,6 +484,7 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 		entry->edx = 0;
 		break;
 	case 7: {
+		int i, times = entry->eax;
 		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
 		/* Mask ebx against host capability word 9 */
 		if (index == 0) {
@@ -507,12 +508,23 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 			 * if the host doesn't support it.
 			 */
 			entry->edx |= F(ARCH_CAPABILITIES);
-		} else {
+		} else if (index > times) {
+			entry->eax = 0;
 			entry->ebx = 0;
 			entry->ecx = 0;
 			entry->edx = 0;
 		}
-		entry->eax = 0;
+		for (i = 1; i <= times; i++) {
+			if (*nent >= maxnent)
+				goto out;
+			do_cpuid_1_ent(&entry[i], function, i);
+			entry[i].eax &= F(AVX512_BF16);
+			entry[i].ebx = 0;
+			entry[i].ecx = 0;
+			entry[i].edx = 0;
+			entry[i].flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
+			++*nent;
+		}
 		break;
 	}
 	case 9:
-- 
1.8.3.1