Re: [PATCH v15 04/21] KVM: x86: Introduce kvm->arch.supports_gmem

[Sean Christopherson <seanjc@google.com>]

On Thu, Jul 17, 2025, Fuad Tabba wrote:
> Introduce a new boolean member, supports_gmem, to kvm->arch.
> 
> Previously, the has_private_mem boolean within kvm->arch was implicitly
> used to indicate whether guest_memfd was supported for a KVM instance.
> However, with the broader support for guest_memfd, it's not exclusively
> for private or confidential memory. Therefore, it's necessary to
> distinguish between a VM's general guest_memfd capabilities and its
> support for private memory.
> 
> This new supports_gmem member will now explicitly indicate guest_memfd
> support for a given VM, allowing has_private_mem to represent only
> support for private memory.
> 
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Reviewed-by: Shivank Garg <shivankg@amd.com>
> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
> Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
> Co-developed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Fuad Tabba <tabba@google.com>

NAK, this introduces unnecessary potential for bugs, e.g. KVM will get a false
negative if kvm_arch_supports_gmem() is invoked before kvm_x86_ops.vm_init().

Patch 2 makes this a moot point because kvm_arch_supports_gmem() can simply go away.

> ---
>  arch/x86/include/asm/kvm_host.h | 3 ++-
>  arch/x86/kvm/svm/svm.c          | 1 +
>  arch/x86/kvm/vmx/tdx.c          | 1 +
>  arch/x86/kvm/x86.c              | 4 ++--
>  4 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index bde811b2d303..938b5be03d33 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1348,6 +1348,7 @@ struct kvm_arch {
>  	u8 mmu_valid_gen;
>  	u8 vm_type;
>  	bool has_private_mem;
> +	bool supports_gmem;
>  	bool has_protected_state;
>  	bool pre_fault_allowed;
>  	struct hlist_head mmu_page_hash[KVM_NUM_MMU_PAGES];
> @@ -2277,7 +2278,7 @@ void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
>  
>  #ifdef CONFIG_KVM_GMEM
>  #define kvm_arch_has_private_mem(kvm) ((kvm)->arch.has_private_mem)
> -#define kvm_arch_supports_gmem(kvm) kvm_arch_has_private_mem(kvm)
> +#define kvm_arch_supports_gmem(kvm)  ((kvm)->arch.supports_gmem)
>  #else
>  #define kvm_arch_has_private_mem(kvm) false
>  #define kvm_arch_supports_gmem(kvm) false
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index ab9b947dbf4f..d1c484eaa8ad 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -5181,6 +5181,7 @@ static int svm_vm_init(struct kvm *kvm)
>  		to_kvm_sev_info(kvm)->need_init = true;
>  
>  		kvm->arch.has_private_mem = (type == KVM_X86_SNP_VM);
> +		kvm->arch.supports_gmem = (type == KVM_X86_SNP_VM);
>  		kvm->arch.pre_fault_allowed = !kvm->arch.has_private_mem;
>  	}
>  
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index f31ccdeb905b..a3db6df245ee 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -632,6 +632,7 @@ int tdx_vm_init(struct kvm *kvm)
>  
>  	kvm->arch.has_protected_state = true;
>  	kvm->arch.has_private_mem = true;
> +	kvm->arch.supports_gmem = true;
>  	kvm->arch.disabled_quirks |= KVM_X86_QUIRK_IGNORE_GUEST_PAT;
>  
>  	/*
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 357b9e3a6cef..adbdc2cc97d4 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12780,8 +12780,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>  		return -EINVAL;
>  
>  	kvm->arch.vm_type = type;
> -	kvm->arch.has_private_mem =
> -		(type == KVM_X86_SW_PROTECTED_VM);
> +	kvm->arch.has_private_mem = (type == KVM_X86_SW_PROTECTED_VM);
> +	kvm->arch.supports_gmem = (type == KVM_X86_SW_PROTECTED_VM);
>  	/* Decided by the vendor code for other VM types.  */
>  	kvm->arch.pre_fault_allowed =
>  		type == KVM_X86_DEFAULT_VM || type == KVM_X86_SW_PROTECTED_VM;
> -- 
> 2.50.0.727.gbf7dc18ff4-goog
>