Re: [PATCH v5.1 27/34] KVM: SVM: Add support for booting APs in an SEV-ES guest

[Tom Lendacky]

On 1/7/21 12:13 PM, Paolo Bonzini wrote:
> On 04/01/21 21:20, Tom Lendacky wrote:
> > From: Tom Lendacky <thomas.lendacky@xxxxxxx>
> >
> > Typically under KVM, an AP is booted using the INIT-SIPI-SIPI sequence,
> > where the guest vCPU register state is updated and then the vCPU is VMRUN
> > to begin execution of the AP. For an SEV-ES guest, this won't work because
> > the guest register state is encrypted.
> >
> > Following the GHCB specification, the hypervisor must not alter the guest
> > register state, so KVM must track an AP/vCPU boot. Should the guest want
> > to park the AP, it must use the AP Reset Hold exit event in place of, for
> > example, a HLT loop.
> >
> > First AP boot (first INIT-SIPI-SIPI sequence):
> >    Execute the AP (vCPU) as it was initialized and measured by the SEV-ES
> >    support. It is up to the guest to transfer control of the AP to the
> >    proper location.
> >
> > Subsequent AP boot:
> >    KVM will expect to receive an AP Reset Hold exit event indicating that
> >    the vCPU is being parked and will require an INIT-SIPI-SIPI sequence to
> >    awaken it. When the AP Reset Hold exit event is received, KVM will place
> >    the vCPU into a simulated HLT mode. Upon receiving the INIT-SIPI-SIPI
> >    sequence, KVM will make the vCPU runnable. It is again up to the guest
> >    to then transfer control of the AP to the proper location.
> >
> >    To differentiate between an actual HLT and an AP Reset Hold, a new MP
> >    state is introduced, KVM_MP_STATE_AP_RESET_HOLD, which the vCPU is
> >    placed in upon receiving the AP Reset Hold exit event. Additionally, to
> >    communicate the AP Reset Hold exit event up to userspace (if needed), a
> >    new exit reason is introduced, KVM_EXIT_AP_RESET_HOLD.
> >
> > A new x86 ops function is introduced, vcpu_deliver_sipi_vector, in order
> > to accomplish AP booting. For VMX, vcpu_deliver_sipi_vector is set to the
> > original SIPI delivery function, kvm_vcpu_deliver_sipi_vector(). SVM adds
> > a new function that, for non SEV-ES guests, invokes the original SIPI
> > delivery function, kvm_vcpu_deliver_sipi_vector(), but for SEV-ES guests,
> > implements the logic above.
> >
> > Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
>
> Queued, thanks.

Thanks, Paolo!

Tom

> Paolo