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

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Message-ID: <CALMp9eSXGjVLXBFmvyVn5RLKOMkVxvVLiR5Sy4abyMiHkeNmdA@mail.gmail.com>
Date:   Fri, 14 Aug 2020 10:34:33 -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 14, 2020 at 3:09 AM Chenyi Qiang <chenyi.qiang@...el.com> wrote:
>
>
>
> On 8/14/2020 1:52 AM, Jim Mattson wrote:
> > On Wed, Aug 12, 2020 at 9:54 PM Chenyi Qiang <chenyi.qiang@...el.com> wrote:
> >>
> >>
> >>
> >> On 8/11/2020 8:05 AM, Jim Mattson wrote:
> >>> 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>
> >>>> ---
> >
> >>>> +           (!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?
> >>>
> >>
> >> We overwrite the L2 PKRS with L1's value when L2 doesn't support PKS.
> >> Because the L1's VM_ENTRY_LOAD_IA32_PKRS is off, we need to migrate L1's
> >> PKRS to L2.
> >
> > I'm thinking of the case where vmx->nested.nested_run_pending is
> > false, and we are processing a KVM_SET_NESTED_STATE ioctl, yet
> > VM_ENTRY_LOAD_IA32_PKRS *is* set in the vmcs12.
> >
>
> Oh, I miss this case. What I'm still confused here is that the
> restoration for GUEST_IA32_DEBUGCTL and GUEST_BNDCFGS have the same
> issue, right? or I miss something.

I take it back. This does work, assuming that userspace calls
KVM_SET_MSRS before calling KVM_SET_NESTED_STATE. Assuming L2 is
active when the checkpoint is taken, the MSR values saved will be the
L2 values. When restoring the MSRs with KVM_SET_MSRS, the L2 MSR
values will be written into vmcs01. They don't belong there, but we're
never going to launch vmcs01 with those MSR values. Instead, when
userspace calls KVM_SET_NESTED_STATE, those values will be transferred
first to the vmcs01_<msr> fields of the vmx->nested struct, and then
to vmcs02.

This is subtle, and I don't think it's documented anywhere that
KVM_SET_NESTED_STATE must be called after KVM_SET_MSRS. In fact, there
are probably a number of dependencies among the various KVM_SET_*
functions that aren't documented anywhere.