linux-kernel - Re: [PATCH 00/10] KVM: SEV: allow customizing VMSA features

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Message-ID: <ZcZ_m5By49jsKNXn@google.com>
Date: Fri, 9 Feb 2024 11:40:11 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, michael.roth@....com, 
	aik@....com, isaku.yamahata@...el.com
Subject: Re: [PATCH 00/10] KVM: SEV: allow customizing VMSA features

On Fri, Feb 09, 2024, Paolo Bonzini wrote:
> The idea that no parameter would ever be necessary when enabling SEV or
> SEV-ES for a VM was decidedly optimistic.

That implies there was a conscious decision regarding the uAPI.  AFAICT, all of
the SEV uAPIs are direct reflections of the PSP invocations.  Which is why I'm
being so draconian about the SNP uAPIs; this time around, we need to actually
design something.

> The first source of variability that was encountered is the desired set of
> VMSA features, as that affects the measurement of the VM's initial state and
> cannot be changed arbitrarily by the hypervisor.
> 
> This series adds all the APIs that are needed to customize the features,
> with room for future enhancements:
> 
> - a new /dev/kvm device attribute to retrieve the set of supported
>   features (right now, only debug swap)
> 
> - a new sub-operation for KVM_MEM_ENCRYPT_OP that can take a struct,
>   replacing the existing KVM_SEV_INIT and KVM_SEV_ES_INIT
> 
> It then puts the new op to work by including the VMSA features as a field
> of the The existing KVM_SEV_INIT and KVM_SEV_ES_INIT use the full set of
> supported VMSA features for backwards compatibility; but I am considering
> also making them use zero as the feature mask, and will gladly adjust the
> patches if so requested.

Rather than add a new KVM_MEMORY_ENCRYPT_OP, I think we should go for broke and
start building the generic set of "protected VM" APIs.  E.g. TDX wants to add
KVM_TDX_INIT_VM, and I'm guessing ARM needs similar functionality.  And AFAIK,
every technology follows an INIT => ADD (MEASURE) * N => FINALIZE type sequence.

If need be, I would rather have a massive union, a la kvm_run, to hold the vendor
specific bits than end up with sub-sub-ioctls and every vendor implementation
reinventing the wheel.

If it's sane and feasible for userspace, maybe even KVM_CREATE_VM2?

> In order to avoid creating *two* new KVM_MEM_ENCRYPT_OPs, I decided that
> I could as well make SEV and SEV-ES use VM types.  And then, why not make
> a SEV-ES VM, when created with the new VM type instead of KVM_SEV_ES_INIT,
> reject KVM_GET_REGS/KVM_SET_REGS and friends on the vCPU file descriptor
> once the VMSA has been encrypted...  Which is how the API should have
> always behaved.

+1000