linux-kernel - Re: [V2 PATCH 1/6] KVM: x86: Add support for testing private memory

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Date:   Tue, 17 Jan 2023 21:39:16 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vishal Annapurve <vannapurve@...gle.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, pbonzini@...hat.com,
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Subject: Re: [V2 PATCH 1/6] KVM: x86: Add support for testing private memory

On Mon, Dec 05, 2022, Vishal Annapurve wrote:
> Introduce HAVE_KVM_PRIVATE_MEM_TESTING config to be able to test fd based
> @@ -272,13 +274,15 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  		.rsvd = err & PFERR_RSVD_MASK,
>  		.user = err & PFERR_USER_MASK,
>  		.prefetch = prefetch,
> -		.is_tdp = likely(vcpu->arch.mmu->page_fault == kvm_tdp_page_fault),
> +		.is_tdp = is_tdp,
>  		.nx_huge_page_workaround_enabled =
>  			is_nx_huge_page_enabled(vcpu->kvm),
>  
>  		.max_level = KVM_MAX_HUGEPAGE_LEVEL,
>  		.req_level = PG_LEVEL_4K,
>  		.goal_level = PG_LEVEL_4K,
> +		.is_private = IS_ENABLED(CONFIG_HAVE_KVM_PRIVATE_MEM_TESTING) && is_tdp &&
> +				kvm_mem_is_private(vcpu->kvm, cr2_or_gpa >> PAGE_SHIFT),

After looking at the SNP+UPM series, I think we should forego a dedicated Kconfig
for testing and instead add a new VM type for UPM-capable guests.  The new type,
e.g. KVM_X86_PROTECTED_VM, can then be used to leverage UPM for "legacy" SEV and
SEV-ES guests, as well as for UPM-capable guests that don't utilize per-VM
memory encryption, e.g. KVM selftests.

Carrying test-only behavior is obviously never ideal, and it would pretty much have
to be mutually exclusive with "real" usage of UPM, otherwise the KVM logics gets
unnecessarily complex.