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

[Jim Mattson <jmattson@google.com>]

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().

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.