Re: [PATCH v16 09/22] KVM: x86: Enable KVM_GUEST_MEMFD for all 64-bit builds

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 7/23/2025 6:47 PM, Fuad Tabba wrote:
> Enable KVM_GUEST_MEMFD for all KVM x86 64-bit builds, i.e. for "default"
> VM types when running on 64-bit KVM.  This will allow using guest_memfd
> to back non-private memory for all VM shapes, by supporting mmap() on
> guest_memfd.
> 
> Opportunistically clean up various conditionals that become tautologies
> once x86 selects KVM_GUEST_MEMFD more broadly.  Specifically, because
> SW protected VMs, SEV, and TDX are all 64-bit only, private memory no
> longer needs to take explicit dependencies on KVM_GUEST_MEMFD, because
> it is effectively a prerequisite.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Fuad Tabba <tabba@google.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>

It matches with my thought on v14 that gmem can be allowed for all x86 
VM types[*]. Get rid of .supports_gmem and achieve it through 
CONFIG_KVM_GUEST_MEMFD looks much cleaner!

[*] 
https://lore.kernel.org/all/b5fe8f54-64df-4cfa-b86f-eed1cbddca7a@intel.com/

> ---
>   arch/x86/include/asm/kvm_host.h |  4 +---
>   arch/x86/kvm/Kconfig            | 12 ++++--------
>   include/linux/kvm_host.h        |  9 ++-------
>   virt/kvm/kvm_main.c             |  4 ++--
>   4 files changed, 9 insertions(+), 20 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 7b0f2b3e492d..50366a1ca192 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -2276,10 +2276,8 @@ void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
>   		       int tdp_max_root_level, int tdp_huge_page_level);
>   
>   
> -#ifdef CONFIG_KVM_GUEST_MEMFD
> +#ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
>   #define kvm_arch_has_private_mem(kvm) ((kvm)->arch.has_private_mem)
> -#else
> -#define kvm_arch_has_private_mem(kvm) false
>   #endif
>   
>   #define kvm_arch_has_readonly_mem(kvm) (!(kvm)->arch.has_protected_state)
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index c763446d9b9f..4e43923656d0 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -47,6 +47,7 @@ config KVM_X86
>   	select KVM_GENERIC_HARDWARE_ENABLING
>   	select KVM_GENERIC_PRE_FAULT_MEMORY
>   	select KVM_WERROR if WERROR
> +	select KVM_GUEST_MEMFD if X86_64
>   
>   config KVM
>   	tristate "Kernel-based Virtual Machine (KVM) support"
> @@ -79,16 +80,11 @@ config KVM_WERROR
>   
>   	  If in doubt, say "N".
>   
> -config KVM_X86_PRIVATE_MEM
> -	select KVM_GENERIC_MEMORY_ATTRIBUTES
> -	select KVM_GUEST_MEMFD
> -	bool
> -
>   config KVM_SW_PROTECTED_VM
>   	bool "Enable support for KVM software-protected VMs"
>   	depends on EXPERT
>   	depends on KVM_X86 && X86_64
> -	select KVM_X86_PRIVATE_MEM
> +	select KVM_GENERIC_MEMORY_ATTRIBUTES
>   	help
>   	  Enable support for KVM software-protected VMs.  Currently, software-
>   	  protected VMs are purely a development and testing vehicle for
> @@ -138,7 +134,7 @@ config KVM_INTEL_TDX
>   	bool "Intel Trust Domain Extensions (TDX) support"
>   	default y
>   	depends on INTEL_TDX_HOST
> -	select KVM_X86_PRIVATE_MEM
> +	select KVM_GENERIC_MEMORY_ATTRIBUTES
>   	select HAVE_KVM_ARCH_GMEM_POPULATE
>   	help
>   	  Provides support for launching Intel Trust Domain Extensions (TDX)
> @@ -162,7 +158,7 @@ config KVM_AMD_SEV
>   	depends on KVM_AMD && X86_64
>   	depends on CRYPTO_DEV_SP_PSP && !(KVM_AMD=y && CRYPTO_DEV_CCP_DD=m)
>   	select ARCH_HAS_CC_PLATFORM
> -	select KVM_X86_PRIVATE_MEM
> +	select KVM_GENERIC_MEMORY_ATTRIBUTES
>   	select HAVE_KVM_ARCH_GMEM_PREPARE
>   	select HAVE_KVM_ARCH_GMEM_INVALIDATE
>   	select HAVE_KVM_ARCH_GMEM_POPULATE
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 56ea8c862cfd..4d1c44622056 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -719,11 +719,7 @@ static inline int kvm_arch_vcpu_memslots_id(struct kvm_vcpu *vcpu)
>   }
>   #endif
>   
> -/*
> - * Arch code must define kvm_arch_has_private_mem if support for guest_memfd is
> - * enabled.
> - */
> -#if !defined(kvm_arch_has_private_mem) && !IS_ENABLED(CONFIG_KVM_GUEST_MEMFD)
> +#ifndef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
>   static inline bool kvm_arch_has_private_mem(struct kvm *kvm)
>   {
>   	return false;
> @@ -2505,8 +2501,7 @@ bool kvm_arch_post_set_memory_attributes(struct kvm *kvm,
>   
>   static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>   {
> -	return IS_ENABLED(CONFIG_KVM_GUEST_MEMFD) &&
> -	       kvm_get_memory_attributes(kvm, gfn) & KVM_MEMORY_ATTRIBUTE_PRIVATE;
> +	return kvm_get_memory_attributes(kvm, gfn) & KVM_MEMORY_ATTRIBUTE_PRIVATE;
>   }
>   #else
>   static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index aa86dfd757db..4f57cb92e109 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1588,7 +1588,7 @@ static int check_memory_region_flags(struct kvm *kvm,
>   {
>   	u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES;
>   
> -	if (kvm_arch_has_private_mem(kvm))
> +	if (IS_ENABLED(CONFIG_KVM_GUEST_MEMFD))
>   		valid_flags |= KVM_MEM_GUEST_MEMFD;
>   
>   	/* Dirty logging private memory is not currently supported. */
> @@ -4917,7 +4917,7 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
>   #endif
>   #ifdef CONFIG_KVM_GUEST_MEMFD
>   	case KVM_CAP_GUEST_MEMFD:
> -		return !kvm || kvm_arch_has_private_mem(kvm);
> +		return 1;
>   #endif
>   	default:
>   		break;