linux-kernel - Re: [RFC PATCH 0/6] KVM: guest memory: Misc enhacnement

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Date:   Wed, 21 Jun 2023 11:16:29 +0300
From:   Zhi Wang <zhi.wang.linux@...il.com>
To:     Vishal Annapurve <vannapurve@...gle.com>
Cc:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com,
        Sean Christopherson <seanjc@...gle.com>,
        Sagi Shahar <sagis@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Kai Huang <kai.huang@...el.com>, chen.bo@...el.com,
        linux-coco@...ts.linux.dev,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Michael Roth <michael.roth@....com>
Subject: Re: [RFC PATCH 0/6] KVM: guest memory: Misc enhacnement

On Mon, 19 Jun 2023 14:55:09 -0700
Vishal Annapurve <vannapurve@...gle.com> wrote:

> On Mon, Jun 19, 2023 at 1:11___PM Zhi Wang <zhi.wang.linux@...il.com> wrote:
> >
> > On Mon, 19 Jun 2023 12:11:50 -0700
> > Vishal Annapurve <vannapurve@...gle.com> wrote:
> >
> > > On Thu, Jun 15, 2023 at 1:12___PM <isaku.yamahata@...el.com> wrote:
> > > > ...
> > > >
> > > > * VM type: Now we have KVM_X86_PROTECTED_VM. How do we proceed?
> > > >   - Keep KVM_X86_PROTECTED_VM for its use. Introduce KVM_X86_TDX_VM
> > > >   - Use KVM_X86_PROTECTED_VM for TDX. (If necessary, introduce another type in
> > > >     the future)
> > > >   - any other way?
> > >
> > > There are selftests posted[1] in context of this work, which rely on
> > > KVM_X86_PROTECTED_VM being just the software-only psuedo-confidential
> > > VMs. In future there might be more work to expand this usecase to
> > > full-scale VMs. So it would be better to treat protected VMs as a
> > > separate type which can be used on any platform without the need of
> > > enabling TDX/SEV functionality.
> > >
> >
> > Out of curiosity, is this really a valid case in practice except selftest?
> > It sounds to me whenever KVM_X86_PROTECTED_VM is used, it has to be tied
> > with a platform-specific CC type.
> 
> Protected VM effort is about being able to have guest memory ranges
> not mapped into Userspace VMM and so are unreachable for most of the
> cases from KVM as well. Non-CC VMs can use this support to mitigate
> any unintended accesses from userspace VMM/KVM possibly using
> enlightened kernels.
> 
> Exact implementation of such a support warrants more discussion but it
> should be in the line of sight here as a future work item.
> 
>

IIUC, what you are saying is (KVM_X86_PROTECTED_VM == (VMs with UPM or GMEM))
&& (KVM_X86_PROTECTED_VM != KVM_X86_CC_VM) && KVM_X86_CC_VM requires
KVM_X86_PROTECTED_VM.

If we think KVM_X86_PROTECTED_VM as a dedicated feature, not tightly coupled
with CC techs, it seems we needs another defination of KVM_X86_CC_VM to represent
CC VM and CC platform types like KVM_X86_CC_TDX_VM to tell which CC tech sits
behind it?

I don't think it is good to mix the usages of KVM_X86_PROTECTED_VM and KVM_X86_CC_VM
together if we are sure KVM_X86_PROTECTED_VM is not going to represent CC VMs
in the code.

> 
> 
> >
> > > TDX VM type can possibly serve as a specialized type of protected VM
> > > with additional arch specific capabilities enabled.
> > >
> > > [1] - https://github.com/sean-jc/linux/commits/x86/kvm_gmem_solo
> >