Re: [PATCH RFC v7 03/64] KVM: SVM: Advertise private memory support to KVM

[Huang, Kai]

On Wed, 2022-12-14 at 13:39 -0600, Michael Roth wrote:
> From: Nikunj A Dadhania <nikunj@xxxxxxx>
> 
> KVM should use private memory for guests that have upm_mode flag set.
> 
> Add a kvm_x86_ops hook for determining UPM support that accounts for
> this situation by only enabling UPM test mode in the case of non-SEV
> guests.
> 
> Signed-off-by: Nikunj A Dadhania <nikunj@xxxxxxx>
> [mdr: add x86 hook for determining restricted/private memory support]
> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
> ---
>  arch/x86/include/asm/kvm-x86-ops.h |  1 +
>  arch/x86/include/asm/kvm_host.h    |  1 +
>  arch/x86/kvm/svm/svm.c             | 10 ++++++++++
>  arch/x86/kvm/x86.c                 |  8 ++++++++
>  4 files changed, 20 insertions(+)
> 
> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> index abccd51dcfca..f530a550c092 100644
> --- a/arch/x86/include/asm/kvm-x86-ops.h
> +++ b/arch/x86/include/asm/kvm-x86-ops.h
> @@ -131,6 +131,7 @@ KVM_X86_OP(msr_filter_changed)
>  KVM_X86_OP(complete_emulated_msr)
>  KVM_X86_OP(vcpu_deliver_sipi_vector)
>  KVM_X86_OP_OPTIONAL_RET0(vcpu_get_apicv_inhibit_reasons);
> +KVM_X86_OP_OPTIONAL_RET0(private_mem_enabled);
>  
>  #undef KVM_X86_OP
>  #undef KVM_X86_OP_OPTIONAL
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 2b6244525107..9317abffbf68 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1635,6 +1635,7 @@ struct kvm_x86_ops {
>  
>  	void (*load_mmu_pgd)(struct kvm_vcpu *vcpu, hpa_t root_hpa,
>  			     int root_level);
> +	int (*private_mem_enabled)(struct kvm *kvm);
>  
>  	bool (*has_wbinvd_exit)(void);
>  
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 91352d692845..7f3e4d91c0c6 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4694,6 +4694,14 @@ static int svm_vm_init(struct kvm *kvm)
>  	return 0;
>  }
>  
> +static int svm_private_mem_enabled(struct kvm *kvm)
> +{
> +	if (sev_guest(kvm))
> +		return kvm->arch.upm_mode ? 1 : 0;
> +
> +	return IS_ENABLED(CONFIG_HAVE_KVM_PRIVATE_MEM_TESTING) ? 1 : 0;
> +}
> +

Is this new callback really needed?  Shouldn't kvm->arch.upm_mode be sufficient
enough to indicate whether the private memory will be used or not?

Probably the CONFIG_HAVE_KVM_PRIVATE_MEM_TESTING is the concern here.  But this
Kconfig option is not even x86-specific, so shouldn't the handling of it be done
in common code too?

For instance, can we explicitly set 'kvm->arch.upm_mode' to 'true' at some point
of creating the VM if we see CONFIG_HAVE_KVM_PRIVATE_MEM_TESTING is true?

[snip]