Re: [PATCH 0/2 V7] Add AMD SEV and SEV-ES intra host migration support

[Sean Christopherson <seanjc@google.com>]

Please Cc the cover letter to anyone that was Cc'd on one or more patches.  That's
especially helpful if some recipients aren't subscribed to KVM.  Oh, and Cc lkml
as well, otherwise I believe lore, patchwork, etc... won't have the cover letter.

On Thu, Sep 02, 2021, Peter Gonda wrote:
> Intra host migration provides a low-cost mechanism for userspace VMM
> upgrades.  It is an alternative to traditional (i.e., remote) live
> migration. Whereas remote migration handles moving a guest to a new host,
> intra host migration only handles moving a guest to a new userspace VMM
> within a host.  This can be used to update, rollback, change flags of the
> VMM, etc. The lower cost compared to live migration comes from the fact
> that the guest's memory does not need to be copied between processes. A
> handle to the guest memory simply gets passed to the new VMM, this could
> be done via /dev/shm with share=on or similar feature.
> 
> The guest state can be transferred from an old VMM to a new VMM as follows:
> 1. Export guest state from KVM to the old user-space VMM via a getter
> user-space/kernel API 2. Transfer guest state from old VMM to new VMM via
> IPC communication 3. Import guest state into KVM from the new user-space
> VMM via a setter user-space/kernel API VMMs by exporting from KVM using
> getters, sending that data to the new VMM, then setting it again in KVM.
> 
> In the common case for intra host migration, we can rely on the normal
> ioctls for passing data from one VMM to the next. SEV, SEV-ES, and other
> confidential compute environments make most of this information opaque, and
> render KVM ioctls such as "KVM_GET_REGS" irrelevant.  As a result, we need
> the ability to pass this opaque metadata from one VMM to the next. The
> easiest way to do this is to leave this data in the kernel, and transfer
> ownership of the metadata from one KVM VM (or vCPU) to the next. For
> example, we need to move the SEV enabled ASID, VMSAs, and GHCB metadata
> from one VMM to the next.  In general, we need to be able to hand off any
> data that would be unsafe/impossible for the kernel to hand directly to
> userspace (and cannot be reproduced using data that can be handed safely to
> userspace).
> 
> For the intra host operation the SEV required metadata, the source VM FD is
> sent to the target VMM. The target VMM calls the new cap ioctl with the
> source VM FD, KVM then moves all the SEV state to the target VM from the
> source VM.