On Wed, Jul 14, 2021 at 08:17:29PM +0000, Sean Christopherson wrote: > On Wed, Jul 07, 2021, Brijesh Singh wrote: > > From: Tom Lendacky <thomas.lendacky@xxxxxxx> > > > > Add support for AP Reset Hold being invoked using the GHCB MSR protocol, > > available in version 2 of the GHCB specification. > > Please provide a brief overview of the protocol, and why it's needed. I assume > it's to allow AP wakeup without a shared GHCB? Yes, this is needed for SEV-ES kexec support to park APs without the need for memory that will be owned by the new kernel when APs are woken up. You can have a look into my SEV-ES kexec/kdump patch-set for details: https://lore.kernel.org/lkml/20210705082443.14721-1-joro@xxxxxxxxxx/ I also sent this patch separatly earlier this week to enable GHCB protocol version 2 support in KVM. Regards, Joerg
