Re: [RFC PATCH v4 11/36] i386/tdx: Initialize TDX before creating TD vcpus

[Xiaoyao Li]

On 5/24/2022 2:57 PM, Gerd Hoffmann wrote:
>    Hi,
>
> > > Hmm, hooking *vm* initialization into *vcpu* creation looks wrong to me.
> >
> > That's because for TDX, it has to do VM-scope (feature) initialization
> > before creating vcpu. This is new to KVM and QEMU, that every feature is
> > vcpu-scope and configured per-vcpu before.
> >
> > To minimize the change to QEMU, we want to utilize @cpu and @cpu->env to
> > grab the configuration info. That's why it goes this way.
> >
> > Do you have any better idea on it?
>
> Maybe it's a bit more work to add VM-scope initialization support to
> qemu.  

If just introducing VM-scope initialization to QEMU, it would be easy. 
What matters is what needs to be done inside VM-scope initialization.

For TDX, we need to settle down the features that configured for the TD. 
Typically, the features are attributes of cpu object, parsed from "-cpu" 
option and stored in cpu object.

I cannot think up a clean solution for it, other than
1) implement the same attributes from cpu object to machine object, or
2) create a CPU object when initializing machine object and collect all 
the info from "-cpu" and drop it in the end; then why not do it when 
creating 1st vcpu like this patch.

That's what I can think up. Let's see if anyone has better idea.

> But I expect that approach will work better long-term.  You need
> this mutex and the 'initialized' variable in your code to make sure it
> runs only once because the way you hook it in is not ideal ...
>
> [ disclaimer: I'm not that familiar with the kvm interface in qemu ]
>
> take care,
>    Gerd