Messages in this thread | | | Subject | Re: [PATCH v2 3/4] Add support for AMD Core Perf Extension in guest | From | "Natarajan, Janakarajan" <> | Date | Mon, 27 Nov 2017 12:21:37 -0600 |
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On 11/17/2017 5:44 AM, Borislav Petkov wrote: > On Thu, Nov 16, 2017 at 12:00:11PM -0600, Natarajan, Janakarajan wrote: >> Ah my apologies. So when the pmu is initialized the cpuid entries >> aren't available then. > So let's see: > > ... kvm_arch_vcpu_create() -> > svm_create_vcpu() -> > kvm_vcpu_init() -> > kvm_arch_vcpu_init() -> > > <--- HERE > > kvm_pmu_init() > > But at HERE in kvm_arch_vcpu_init() right before kvm_pmu_init() we do already query > cpuid: > > vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);
cpuid_query_maxphyaddr first checks for for the 0x80000008 cpuid entry, and if it can't find it returns a default value of 36.
When invoked in the kvm_arch_vcpu_init(), the default is being returned. The vcpu->arch.maxphyaddr is later updated in the kvm_update_cpuid(), which in my case was setting the maxphyaddr to 40. I believe the cpuid entry is not available when invoked in the init call and a default value is being used as a placeholder until the entries are updated by qemu.
> so it's not like we don't know about cpuid leafs at that point. Which > would mean that the code can be made to set the CPU family earlier, > before kvm_pmu_init() runs so that you have the proper CPU family and > thus have this thing properly designed. > > Maybe Paolo and Radim have a better suggestion here... >
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