Shortlog is too long, maybe this?
KVM: nSVM: Raise event on nested VM exit if L1 doesn't intercept IRQs
On Tue, Nov 29, 2022, Maxim Levitsky wrote:
> If the L2 doesn't intercept interrupts, then the KVM will use vmcb02's
s/the L2/L2, though don't you mean L1?
> V_IRQ for L1 (to detect the interrupt window)
"an interrupt window", i.e. there's not just one window.
> In this case on the nested VM exit KVM might need to copy the V_IRQ bit
s/the nested/nested
> from the vmcb02 to the vmcb01, to continue waiting for the
> interrupt window.
>
> To make it simple, just raise the KVM_REQ_EVENT request, which
> execution will lead to the reenabling of the interrupt
> window if needed.
>
> Note that this is a theoretical bug because the KVM already does raise
s/the KVM/KVM
> the KVM_REQ_EVENT request one each nested VM exit because the nested
s/the KVM_REQ_EVENT/KVM_REQ_EVENT, and s/one/on
> VM exit resets RFLAGS and the kvm_set_rflags() raises the
> KVM_REQ_EVENT request in the response.
>
> However raising this request explicitly, together with
> documenting why this is needed, is still preferred.
>
> Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> ---
> arch/x86/kvm/svm/nested.c | 25 +++++++++++++++++++++++++
> 1 file changed, 25 insertions(+)
>
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index aad3145b2f62fe..e891318595113e 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -1016,6 +1016,31 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
>
> svm_switch_vmcb(svm, &svm->vmcb01);
>
> + /* Note about synchronizing some of int_ctl bits from vmcb02 to vmcb01:
/*
* Preferred block comment style...
> + *
> + * - V_IRQ, V_IRQ_VECTOR, V_INTR_PRIO_MASK, V_IGN_TPR:
Drop the "-" to be consistent with the rest of the paragraphs.
> + * If the L2 doesn't intercept interrupts, then
> + * (even if the L2 does use virtual interrupt masking),
KVM uses "L2" to refer to the thing running at L2. I think what you are referring
to here is vmcb12? And that's controlled by L1.
> + * KVM will use the vmcb02's V_INTR to detect interrupt window.
s/the vmcb02/vmcb02
Which of the V_INTR fields does this refer to? Oooh, you're saying the KVM injects
a virtual interrupt into L2 using vmcb02 in order to determine when L2 has IRQs
enabled.
Why does KVM do that? Why not pend the actual IRQ directly?
> + *
> + * In this case, the KVM raises the KVM_REQ_EVENT to ensure that interrupt window
s/the KVM_REQ_EVENT/KVM_REQ_EVENT
> + * is not lost and this implicitly copies these bits from vmcb02 to vmcb01
Too many pronouns. What do "this" and "these bits" refer to?
> + *
> + * V_TPR:
> + * If the L2 doesn't use virtual interrupt masking, then the L1's vTPR
> + * is stored in the vmcb02 but its value doesn't need to be copied from/to
> + * vmcb01 because it is copied from/to the TPR APIC's register on
> + * each VM entry/exit.
> + *
> + * V_GIF:
> + * - If the nested vGIF is not used, KVM uses vmcb02's V_GIF for L1's V_GIF,
Drop this "-" too.
> + * however, the L1 vGIF is reset to false on each VM exit, thus
> + * there is no need to copy it from vmcb02 to vmcb01.
> + */
> +
> + if (!nested_exit_on_intr(svm))
> + kvm_make_request(KVM_REQ_EVENT, &svm->vcpu);
> +
> if (unlikely(svm->lbrv_enabled && (svm->nested.ctl.virt_ext & LBR_CTL_ENABLE_MASK))) {
> svm_copy_lbrs(vmcb12, vmcb02);
> svm_update_lbrv(vcpu);
> --
> 2.26.3
>
