linux-kernel - Re: [PATCH v5 07/17] KVM: x86: add fields to struct kvm

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Date: Tue, 9 Apr 2024 16:43:59 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Rick P Edgecombe <rick.p.edgecombe@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"michael.roth@....com" <michael.roth@....com>, Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [PATCH v5 07/17] KVM: x86: add fields to struct kvm_arch for CoCo features

On Tue, Apr 9, 2024 at 3:21 AM Sean Christopherson <seanjc@...gle.com> wrote:
> > I'm a little late to this conversation, so hopefully not just complicating
> > things. But why not deduce has_private_mem and has_protected_state from the
> > vm_type during runtime? Like if kvm.arch.vm_type was instead a bit mask with
> > the bit position of the KVM_X86_*_VM set, kvm_arch_has_private_mem() could
> > bitwise-and with a compile time mask of vm_types that have primate memory.
> > This also prevents it from ever transitioning through non-nonsensical states
> > like vm_type == KVM_X86_TDX_VM, but !has_private_memory, so would be a little
> > more robust.
>
> LOL, time is a circle, or something like that.  Paolo actually did this in v2[*],
> and I objected, vociferously.

To be fair, Rick is asking for something much less hideous - just set

 kvm->arch.vm_type = (1 << vm_type);

and then define kvm_has_*(kvm) as !!(kvm->arch.vm_type & SOME_BIT_MASK).

And indeed it makes sense as an alternative. It also feels a little
bit more restrictive and the benefit is small, so I think I'm going to
go with this version.

Paolo