Re: [RFC PATCH v2 03/21] HostMem: Add private property and associate it with RAM_KVM_GMEM

[Markus Armbruster <armbru@redhat.com>]

David Hildenbrand <david@redhat.com> writes:

> On 20.09.23 16:35, Xiaoyao Li wrote:
>> On 9/20/2023 3:30 PM, Markus Armbruster wrote:
>>> Xiaoyao Li <xiaoyao.li@intel.com> writes:
>>>
>>>> On 9/19/2023 5:46 PM, Markus Armbruster wrote:
>>>>> Xiaoyao Li <xiaoyao.li@intel.com> writes:
>>>>>
>>>>>> From: Isaku Yamahata <isaku.yamahata@intel.com>
>>>>>>
>>>>>> Add a new property "private" to memory backends. When it's set to true,
>>>>>> it indicates the RAMblock of the backend also requires kvm gmem.
>>>>> Can you add a brief explanation why you need the property?
>>>>
>>>> It provides a mechanism for user to specify whether the memory can serve as private memory (need request kvm gmem).
>>>
>>> Yes, but why would a user want such memory?
>>>
>> Because KVM demands it for confidential guest, e.g., TDX guest. KVM
>> demands that the mem slot needs to have KVM_MEM_PRIVATE set and has
>> valid gmem associated if the guest accesses it as private memory.

Commit messages should explain why we want the patch.  Documenting "why"
is at least as important as "what".  If "what" is missing, I can read
the patch to find out.  If "why" is missing, I'm reduced to guesswork.

> I think as long as there is no demand to have a TDX guest with this property be set to "off", then just don't add it.
>
> With a TDX VM, it will can be implicitly active. If we ever have to disable it for selective memory backends, we can add the property and have something like on/off/auto. For now it would be "auto".

Makes sense to me.