linux-kernel - Re: [RFC 7/7] KVM: VMX: Enable PKS for nested VM

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Date:   Mon, 10 Aug 2020 17:05:36 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Chenyi Qiang <chenyi.qiang@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 7/7] KVM: VMX: Enable PKS for nested VM

On Fri, Aug 7, 2020 at 1:47 AM Chenyi Qiang <chenyi.qiang@...el.com> wrote:
>
> PKS MSR passes through guest directly. Configure the MSR to match the
> L0/L1 settings so that nested VM runs PKS properly.
>
> Signed-off-by: Chenyi Qiang <chenyi.qiang@...el.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 32 ++++++++++++++++++++++++++++++++
>  arch/x86/kvm/vmx/vmcs12.c |  2 ++
>  arch/x86/kvm/vmx/vmcs12.h |  6 +++++-
>  arch/x86/kvm/vmx/vmx.c    | 10 ++++++++++
>  arch/x86/kvm/vmx/vmx.h    |  1 +
>  5 files changed, 50 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index df2c2e733549..1f9823d21ecd 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -647,6 +647,12 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
>                                         MSR_IA32_PRED_CMD,
>                                         MSR_TYPE_W);
>
> +       if (!msr_write_intercepted_l01(vcpu, MSR_IA32_PKRS))
> +               nested_vmx_disable_intercept_for_msr(
> +                                       msr_bitmap_l1, msr_bitmap_l0,
> +                                       MSR_IA32_PKRS,
> +                                       MSR_TYPE_R | MSR_TYPE_W);

What if L1 intercepts only *reads* of MSR_IA32_PKRS?

>         kvm_vcpu_unmap(vcpu, &to_vmx(vcpu)->nested.msr_bitmap_map, false);
>
>         return true;

> @@ -2509,6 +2519,11 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
>         if (kvm_mpx_supported() && (!vmx->nested.nested_run_pending ||
>             !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_BNDCFGS)))
>                 vmcs_write64(GUEST_BNDCFGS, vmx->nested.vmcs01_guest_bndcfgs);
> +
> +       if (kvm_cpu_cap_has(X86_FEATURE_PKS) &&

Is the above check superfluous? I would assume that the L1 guest can't
set VM_ENTRY_LOAD_IA32_PKRS unless this is true.

> +           (!vmx->nested.nested_run_pending ||
> +            !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_IA32_PKRS)))
> +               vmcs_write64(GUEST_IA32_PKRS, vmx->nested.vmcs01_guest_pkrs);

This doesn't seem right to me. On the target of a live migration, with
L2 active at the time the snapshot was taken (i.e.,
vmx->nested.nested_run_pending=0), it looks like we're going to try to
overwrite the current L2 PKRS value with L1's PKRS value (except that
in this situation, vmx->nested.vmcs01_guest_pkrs should actually be
0). Am I missing something?

>         vmx_set_rflags(vcpu, vmcs12->guest_rflags);
>
>         /* EXCEPTION_BITMAP and CR0_GUEST_HOST_MASK should basically be the


> @@ -3916,6 +3943,8 @@ static void sync_vmcs02_to_vmcs12_rare(struct kvm_vcpu *vcpu,
>                 vmcs_readl(GUEST_PENDING_DBG_EXCEPTIONS);
>         if (kvm_mpx_supported())
>                 vmcs12->guest_bndcfgs = vmcs_read64(GUEST_BNDCFGS);
> +       if (kvm_cpu_cap_has(X86_FEATURE_PKS))

Shouldn't we be checking to see if the *virtual* CPU supports PKS
before writing anything into vmcs12->guest_ia32_pkrs?

> +               vmcs12->guest_ia32_pkrs = vmcs_read64(GUEST_IA32_PKRS);
>
>         vmx->nested.need_sync_vmcs02_to_vmcs12_rare = false;
>  }