x86: Enumerate AVX512 FP16 CPUID feature flag

Enumerate AVX512 Half-precision floating point (FP16) CPUID feature
flag. Compared with using FP32, using FP16 cut the number of bits
required for storage in half, reducing the exponent from 8 bits to 5,
and the mantissa from 23 bits to 10. Using FP16 also enables developers
to train and run inference on deep learning models fast when all
precision or magnitude (FP32) is not needed.

A processor supports AVX512 FP16 if CPUID.(EAX=7,ECX=0):EDX[bit 23]
is present. The AVX512 FP16 requires AVX512BW feature be implemented
since the instructions for manipulating 32bit masks are associated with
AVX512BW.

The only in-kernel usage of this is kvm passthrough. The CPU feature
flag is shown as "avx512_fp16" in /proc/cpuinfo.

Signed-off-by: Kyung Min Park <kyung.min.park@intel.com>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Message-Id: <20201208033441.28207-2-kyung.min.park@intel.com>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index dad350d42ecfbf3063c64138375e81591729b092..b9dc6a56d360afc312caea86c75cd6b130d24616 100644 (file)
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -374,6 +374,7 @@
 #define X86_FEATURE_TSXLDTRK           (18*32+16) /* TSX Suspend Load Address Tracking */
 #define X86_FEATURE_PCONFIG            (18*32+18) /* Intel PCONFIG */
 #define X86_FEATURE_ARCH_LBR           (18*32+19) /* Intel ARCH LBR */
+#define X86_FEATURE_AVX512_FP16                (18*32+23) /* AVX512 FP16 */
 #define X86_FEATURE_SPEC_CTRL          (18*32+26) /* "" Speculation Control (IBRS + IBPB) */
 #define X86_FEATURE_INTEL_STIBP                (18*32+27) /* "" Single Thread Indirect Branch Predictors */
 #define X86_FEATURE_FLUSH_L1D          (18*32+28) /* Flush L1D cache */

diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index d502241995a39df7f82ab16a723146d0eecb30d8..42af31b64c2ceb3f96cebd2bb54827b0f084b445 100644 (file)
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -69,6 +69,7 @@ static const struct cpuid_dep cpuid_deps[] = {
        { X86_FEATURE_CQM_MBM_TOTAL,            X86_FEATURE_CQM_LLC   },
        { X86_FEATURE_CQM_MBM_LOCAL,            X86_FEATURE_CQM_LLC   },
        { X86_FEATURE_AVX512_BF16,              X86_FEATURE_AVX512VL  },
+       { X86_FEATURE_AVX512_FP16,              X86_FEATURE_AVX512BW  },
        { X86_FEATURE_ENQCMD,                   X86_FEATURE_XSAVES    },
        { X86_FEATURE_PER_THREAD_MBA,           X86_FEATURE_MBA       },
        {}