Re: [RFC PATCH v2 16/32] x86: kvm: Provide support to create Guest and HV shared per-CPU variables

[Paolo Bonzini]

On 28/03/2017 20:39, Borislav Petkov wrote:
>> 2) Since the encryption attributes works on PAGE_SIZE hence add some extra
>> padding to 'struct kvm-steal-time' to make it PAGE_SIZE and then at runtime
>> clear the encryption attribute of the full PAGE. The downside of this was
>> now we need to modify structure which may break the compatibility.
> From SEV-ES whitepaper:
> 
> "To facilitate this communication, the SEV-ES architecture defines
> a Guest Hypervisor Communication Block (GHCB). The GHCB resides in
> page of shared memory so it is accessible to both the guest VM and the
> hypervisor."
> 
> So this is kinda begging to be implemented with a shared page between
> guest and host. And then put steal-time, ... etc in there too. Provided
> there's enough room in the single page for the GHCB *and* our stuff.

The GHCB would have to be allocated much earlier, possibly even by
firmware depending on how things will be designed.  I think it's
premature to consider SEV-ES requirements.

Paolo