Re: [PATCH 6/8][KVM nVMX]: Load IA32_PERF_GLOBAL_CTRL MSR on vmentry of nested guests

[Krish Sadhukhan <krish.sadhukhan@oracle.com>]

On 8/15/19 3:44 PM, Jim Mattson wrote:
> On Wed, Apr 24, 2019 at 4:43 PM Krish Sadhukhan
> <krish.sadhukhan@oracle.com> wrote:
>> According to section "Loading Guest State" in Intel SDM vol 3C, the
>> IA32_PERF_GLOBAL_CTRL MSR is loaded on vmentry of nested guests:
>>
>>      "If the “load IA32_PERF_GLOBAL_CTRL” VM-entry control is 1, the
>>       IA32_PERF_GLOBAL_CTRL MSR is loaded from the IA32_PERF_GLOBAL_CTRL
>>       field."
>>
>> Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
>> Suggested-by: Jim Mattson <jmattson@google.com>
>> Reviewed-by: Karl Heubaum <karl.heubaum@oracle.com>
>> ---
>>   arch/x86/kvm/vmx/nested.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>> index a7bf19eaa70b..8177374886a9 100644
>> --- a/arch/x86/kvm/vmx/nested.c
>> +++ b/arch/x86/kvm/vmx/nested.c
>> @@ -2300,6 +2300,10 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
>>          vcpu->arch.cr0_guest_owned_bits &= ~vmcs12->cr0_guest_host_mask;
>>          vmcs_writel(CR0_GUEST_HOST_MASK, ~vcpu->arch.cr0_guest_owned_bits);
>>
>> +       if (vmcs12->vm_entry_controls & VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL)
>> +               vmcs_write64(GUEST_IA32_PERF_GLOBAL_CTRL,
>> +                            vmcs12->guest_ia32_perf_global_ctrl);
>> +
>>          if (vmx->nested.nested_run_pending &&
>>              (vmcs12->vm_entry_controls & VM_ENTRY_LOAD_IA32_PAT)) {
>>                  vmcs_write64(GUEST_IA32_PAT, vmcs12->guest_ia32_pat);
>> --
>> 2.17.2
>>
> This isn't quite right. The GUEST_IA32_PERF_GLOBAL_CTRL value is just
> going to get overwritten later by atomic_switch_perf_msrs().


atomic_switch_perf_msrs() gest called from vmx_vcpu_run() which I 
thought was executing before handle_vmlaunch() stuff that lead to 
prepare_vmcs02(). Did I miss something in the call-chain ?


> Instead of writing the vmcs12 value directly into the vmcs02, you
> should call kvm_set_msr(), exactly as it would have been called if
> MSR_CORE_PERF_GLOBAL_CTRL had been in the vmcs12
> VM-entry MSR-load list. Then, atomic_switch_perf_msrs() will
> automatically do the right thing.