Re: [RFC PATCH v2 32/32] x86: kvm: Pin the guest memory when SEV is active

[Brijesh Singh <brijesh.singh@amd.com>]

On 03/16/2017 05:38 AM, Paolo Bonzini wrote:
>
>
> On 02/03/2017 16:18, Brijesh Singh wrote:
>> The SEV memory encryption engine uses a tweak such that two identical
>> plaintexts at different location will have a different ciphertexts.
>> So swapping or moving ciphertexts of two pages will not result in
>> plaintexts being swapped. Relocating (or migrating) a physical backing pages
>> for SEV guest will require some additional steps. The current SEV key
>> management spec [1] does not provide commands to swap or migrate (move)
>> ciphertexts. For now we pin the memory allocated for the SEV guest. In
>> future when SEV key management spec provides the commands to support the
>> page migration we can update the KVM code to remove the pinning logical
>> without making any changes into userspace (qemu).
>>
>> The patch pins userspace memory when a new slot is created and unpin the
>> memory when slot is removed.
>>
>> [1] http://support.amd.com/TechDocs/55766_SEV-KM%20API_Spec.pdf
>
> This is not enough, because memory can be hidden temporarily from the
> guest and remapped later.  Think of a PCI BAR that is backed by RAM, or
> also SMRAM.  The pinning must be kept even in that case.
>
> You need to add a pair of KVM_MEMORY_ENCRYPT_OPs (one that doesn't map
> to a PSP operation), such as KVM_REGISTER/UNREGISTER_ENCRYPTED_RAM.  In
> QEMU you can use a RAMBlockNotifier to invoke the ioctls.
>

I was hoping to avoid adding new ioctl, but I see your point. Will add a pair of ioctl's
and use RAMBlocNotifier to invoke those ioctls.

-Brijesh