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Re: [Qemu-devel] [PATCH v1] x86: Intel AVX512_BF16 feature enabling


From: Jing Liu
Subject: Re: [Qemu-devel] [PATCH v1] x86: Intel AVX512_BF16 feature enabling
Date: Wed, 24 Jul 2019 20:05:38 +0800
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0



On 7/22/2019 7:50 PM, Paolo Bonzini wrote:
On 22/07/19 04:59, Jing Liu wrote:


On 7/19/2019 4:10 PM, Paolo Bonzini wrote:
On 19/07/19 09:20, Jing Liu wrote:
Then CPUID[7,0].EAX is set automatically to 0 or 1 depending on whether
BF16 is enabled or not.

Could I ask why don't we directly check BF16 enabling when
cpu_x86_cpuid(env, 7, 0, ...) during kvm_arch_init_vcpu ?

Because the code for setting CPUID is common for all accelerators (there
are five supported: KVM, HAX, HVF, TCG, WHPX).

What is the use of the two new properties? Are they used for users
setting parameters when boot up guest, and why we need users setting
func7 level?

For example to test guests with CPUID[7,0].EAX==1, even if the host does
not have BF16.

Thanks. :)


@@ -5075,6 +5101,10 @@ static void x86_cpu_expand_features(X86CPU *cpu,
Error **errp)
           x86_cpu_adjust_feat_level(cpu, FEAT_SVM);
           x86_cpu_adjust_feat_level(cpu, FEAT_XSAVE);

+       if ((env->features[FEAT_7_1_EAX] & CPUID_7_1_EAX_AVX512_BF16) &&
+            kvm_enabled()) {

No need to check KVM.  You could also do just
x86_cpu_adjust_feat_level(cpu, FEAT_7_1_EAX) and set
cpu->min_level_func7 in x86_cpu_adjust_feat_level with something like

      if (eax == 7) {
          x86_cpu_adjust_level(cpu, &env->cpu_min_level_func7,
                               fi->cpuid.ecx);
      }


Got it. One question I'm wondering is, is it possible for users setting
an invalid property like level-func7=2? Do we need some protection?

No, it's still not found in Intel silicon, but in principle you could
have higher indices than 1.  So it's okay, if something breaks it's the
fault of whoever set the option!


Thanks very much. So would you like me to update the patch with v2 now?

Jing




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