Re: [PATCH v15 19/21] KVM: Introduce the KVM capability KVM_CAP_GMEM_MMAP

[Sean Christopherson <seanjc@google.com>]

On Thu, Jul 17, 2025, Fuad Tabba wrote:
> Introduce the new KVM capability KVM_CAP_GMEM_MMAP. This capability
> signals to userspace that a KVM instance supports host userspace mapping
> of guest_memfd-backed memory.
> 
> The availability of this capability is determined per architecture, and
> its enablement for a specific guest_memfd instance is controlled by the
> GUEST_MEMFD_FLAG_MMAP flag at creation time.
> 
> Update the KVM API documentation to detail the KVM_CAP_GMEM_MMAP
> capability, the associated GUEST_MEMFD_FLAG_MMAP, and provide essential
> information regarding support for mmap in guest_memfd.
> 
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Reviewed-by: Shivank Garg <shivankg@amd.com>
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>  Documentation/virt/kvm/api.rst | 9 +++++++++
>  include/uapi/linux/kvm.h       | 1 +
>  virt/kvm/kvm_main.c            | 4 ++++
>  3 files changed, 14 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 43ed57e048a8..5169066b53b2 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6407,6 +6407,15 @@ most one mapping per page, i.e. binding multiple memory regions to a single
>  guest_memfd range is not allowed (any number of memory regions can be bound to
>  a single guest_memfd file, but the bound ranges must not overlap).
>  
> +When the capability KVM_CAP_GMEM_MMAP is supported, the 'flags' field supports
> +GUEST_MEMFD_FLAG_MMAP.  Setting this flag on guest_memfd creation enables mmap()
> +and faulting of guest_memfd memory to host userspace.
> +
> +When the KVM MMU performs a PFN lookup to service a guest fault and the backing
> +guest_memfd has the GUEST_MEMFD_FLAG_MMAP set, then the fault will always be
> +consumed from guest_memfd, regardless of whether it is a shared or a private
> +fault.
> +
>  See KVM_SET_USER_MEMORY_REGION2 for additional details.
>  
>  4.143 KVM_PRE_FAULT_MEMORY
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 3beafbf306af..698dd407980f 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -960,6 +960,7 @@ struct kvm_enable_cap {
>  #define KVM_CAP_ARM_EL2 240
>  #define KVM_CAP_ARM_EL2_E2H0 241
>  #define KVM_CAP_RISCV_MP_STATE_RESET 242
> +#define KVM_CAP_GMEM_MMAP 243

KVM_CAP_GUEST_MEMFD_MMAP please.  I definitely don't want "gmem" in any of the
uAPI.

>  struct kvm_irq_routing_irqchip {
>  	__u32 irqchip;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 46bddac1dacd..f1ac872e01e9 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4916,6 +4916,10 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
>  #ifdef CONFIG_KVM_GMEM
>  	case KVM_CAP_GUEST_MEMFD:
>  		return !kvm || kvm_arch_supports_gmem(kvm);
> +#endif
> +#ifdef CONFIG_KVM_GMEM_SUPPORTS_MMAP
> +	case KVM_CAP_GMEM_MMAP:

As alluded to in my feedback in patch 2, this should be inside CONFIG_KVM_GUEST_MEMFD.

> +		return !kvm || kvm_arch_supports_gmem_mmap(kvm);
>  #endif
>  	default:
>  		break;
> -- 
> 2.50.0.727.gbf7dc18ff4-goog
>