Re: [PATCH 04/13] KVM: nSVM: clean up copying of int_ctl fields back to vmcb01/vmcb12

Maxim Levitsky
2022-11-21T11:16:41Z

On Thu, 2022-11-17 at 20:15 +0000, Sean Christopherson wrote:
> On Thu, Nov 17, 2022, Maxim Levitsky wrote:
> > Clean up the nested_sync_int_ctl_from_vmcb02:
> >
> > 1. The comment about preservation of V_IRQ is wrong: when the L2 doesn't
> >    use virtual interrupt masking, then the field just doesn't exist in
> >    vmcb12 thus it should not be touched at all.
> >    Since it is unused in this case, touching it doesn't matter that much,
> >    so the bug is theoretical.
> >
> > 2. When the L2 doesn't use virtual interrupt masking, then in the *theory*
> >    if KVM uses the feature, it should copy the changes to V_IRQ* bits from
> >    vmcb02 to vmcb01.
> >
> >    In practise, KVM only uses it for detection of the interrupt window,
> >    and it happens to re-open it on each nested VM exit because
> >    kvm_set_rflags happens to raise the KVM_REQ_EVENT.
> >    Do this explicitly.
> >
> > 3. Add comment on why we don't need to copy V_GIF from vmcb02 to vmcb01
> >    when nested guest doesn't use nested V_GIF (and thus L1's GIF is in
> >    vmcb02 while nested), even though it can in theory affect L1's GIF.
> >
> > 4. Add support code to also copy some bits of int_ctl from
> >    vmcb02 to vmcb01.
> >    Currently there are none.
>
> Unless it's impossible for whatever reason, this patch should be split into
> multiple patches.  IIUC, there are at least 2 different functional changes being
> made, they just happen to not have any actual impact on things.

No objection to this.

>
> > No (visible) functional change is intended.
> >
> > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> > ---
> >  arch/x86/kvm/svm/nested.c | 47 ++++++++++++++++++++++++++-------------
> >  1 file changed, 32 insertions(+), 15 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > index 54eb152e2b60b6..1f2b8492c8782f 100644
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -410,28 +410,45 @@ void nested_copy_vmcb_save_to_cache(struct vcpu_svm *svm,
> >  static void nested_sync_int_ctl_from_vmcb02(struct vcpu_svm *svm,
> >                                             struct vmcb *vmcb12)
> >  {
> > -       u32 mask;
> > +       struct vmcb *vmcb02 = svm->nested.vmcb02.ptr;
> > +       struct vmcb *vmcb01 = svm->vmcb01.ptr;
> > +
> > +       /* bitmask of bits of int_ctl that we copy from vmcb02 to vmcb12*/
> > +       u32 l2_to_l1_mask = 0;
> > +       /* bitmask of bits of int_ctl that we copy from vmcb02 to vmcb01*/
> > +       u32 l2_to_l0_mask = 0;
> >
> > -       /* Only a few fields of int_ctl are written by the processor.  */
>
> Can this comment be kept in some form?  I found it super useful when reading this
> code just now.

No problem.

>
> > -       mask = V_IRQ_MASK | V_TPR_MASK;
> > -       if (!(svm->nested.ctl.int_ctl & V_INTR_MASKING_MASK) &&
> > -           svm_is_intercept(svm, INTERCEPT_VINTR)) {
> > +       if (svm->nested.ctl.int_ctl & V_INTR_MASKING_MASK)
> > +               l2_to_l1_mask |= V_IRQ_MASK | V_TPR_MASK;
> > +       else {
> >                 /*
> > -                * In order to request an interrupt window, L0 is usurping
> > -                * svm->vmcb->control.int_ctl and possibly setting V_IRQ
> > -                * even if it was clear in L1's VMCB.  Restoring it would be
> > -                * wrong.  However, in this case V_IRQ will remain true until
> > -                * interrupt_window_interception calls svm_clear_vintr and
> > -                * restores int_ctl.  We can just leave it aside.
> > +                * If IRQ window was opened while in L2, it must be reopened
> > +                * after the VM exit
> > +                *
> > +                * vTPR value doesn't need to be copied from vmcb02 to vmcb01
> > +                * because it is synced from/to apic registers on each VM exit
> >                  */
> > -               mask &= ~V_IRQ_MASK;
> > +               if (vmcb02->control.int_ctl & V_IRQ_MASK)
> > +                       kvm_make_request(KVM_REQ_EVENT, &svm->vcpu);
> >         }
> >
> >         if (nested_vgif_enabled(svm))
> > -               mask |= V_GIF_MASK;
> > +               l2_to_l1_mask |= V_GIF_MASK;
> > +       else
> > +               /* There is no need to sync V_GIF from vmcb02 to vmcb01
> > +                * because GIF is cleared on VMexit, thus even though
> > +                * nested guest can control host's GIF, on VM exit
> > +                * its set value is lost
> > +                */
> > +               ;
>
> The "else ... ;" is unnecessary, just throw the block comment above the nested
> vGIF if-statment, e.g. if I'm understanding everything, this?
Yes.

>
>         /*
>          * If nested vGIF is not enabled, L2 has access to L1's "real" GIF.  In
>          * this case, there's no need to sync V_GIF from vmcb02 to vmcb01
>          * because GIF is cleared on VM-Exit, thus any changes made by L2 are
>          * overwritten on VM-Exit to L1.
>          */
>         if (nested_vgif_enabled(svm))
>                 l2_to_l1_mask |= V_GIF_MASK;
>
> > +
> > +       vmcb12->control.int_ctl =
> > +               (svm->nested.ctl.int_ctl & ~l2_to_l1_mask) |
> > +               (vmcb02->control.int_ctl & l2_to_l1_mask);
> >
> > -       vmcb12->control.int_ctl        &= ~mask;
> > -       vmcb12->control.int_ctl        |= svm->vmcb->control.int_ctl & mask;
> > +       vmcb01->control.int_ctl =
> > +               (vmcb01->control.int_ctl & ~l2_to_l0_mask) |
> > +               (vmcb02->control.int_ctl & l2_to_l0_mask);
>
> No need for wrapping immediately after the "=", these all fit under the soft limit:
>
>         vmcb12->control.int_ctl = (svm->nested.ctl.int_ctl & ~l2_to_l1_mask) |
>                                   (vmcb02->control.int_ctl & l2_to_l1_mask);
>
>         vmcb01->control.int_ctl = (vmcb01->control.int_ctl & ~l2_to_l0_mask) |
>                                   (vmcb02->control.int_ctl & l2_to_l0_mask);

OK.

Best regards,
Maxim Levitsky

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