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

Sean Christopherson
2022-11-17T20:15:46Z

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 (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.

> - 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?

/*
* 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);