Re: [PATCH v15 18/21] KVM: arm64: Enable host mapping of shared guest_memfd memory

[Marc Zyngier <maz@kernel.org>]

On Thu, 17 Jul 2025 17:27:28 +0100,
Fuad Tabba <tabba@google.com> wrote:
> 
> Enable host userspace mmap support for guest_memfd-backed memory on
> arm64. This change provides arm64 with the capability to map guest
> memory at the host directly from guest_memfd:
> 
> * Define kvm_arch_supports_gmem_mmap() for arm64: The
>   kvm_arch_supports_gmem_mmap() macro is defined for arm64 to be true if
>   CONFIG_KVM_GMEM_SUPPORTS_MMAP is enabled. For existing arm64 KVM VM
>   types that support guest_memfd, this enables them to use guest_memfd
>   with host userspace mappings. This provides a consistent behavior as
>   there are currently no arm64 CoCo VMs that rely on guest_memfd solely
>   for private, non-mappable memory. Future arm64 VM types can override
>   or restrict this behavior via the kvm_arch_supports_gmem_mmap() hook
>   if needed.
> 
> * Select CONFIG_KVM_GMEM_SUPPORTS_MMAP in arm64 Kconfig.
> 
> * Enforce KVM_MEMSLOT_GMEM_ONLY for guest_memfd on arm64: Checks are
>   added to ensure that if guest_memfd is enabled on arm64,
>   KVM_GMEM_SUPPORTS_MMAP must also be enabled. This means
>   guest_memfd-backed memory slots on arm64 are currently only supported
>   if they are intended for shared memory use cases (i.e.,
>   kvm_memslot_is_gmem_only() is true). This design reflects the current
>   arm64 KVM ecosystem where guest_memfd is primarily being introduced
>   for VMs that support shared memory.
> 
> Reviewed-by: James Houghton <jthoughton@google.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>  arch/arm64/include/asm/kvm_host.h | 4 ++++
>  arch/arm64/kvm/Kconfig            | 2 ++
>  arch/arm64/kvm/mmu.c              | 7 +++++++
>  3 files changed, 13 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 3e41a880b062..63f7827cfa1b 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -1674,5 +1674,9 @@ void compute_fgu(struct kvm *kvm, enum fgt_group_id fgt);
>  void get_reg_fixed_bits(struct kvm *kvm, enum vcpu_sysreg reg, u64 *res0, u64 *res1);
>  void check_feature_map(void);
>  
> +#ifdef CONFIG_KVM_GMEM
> +#define kvm_arch_supports_gmem(kvm) true
> +#define kvm_arch_supports_gmem_mmap(kvm) IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP)
> +#endif

nit: these two lines should be trivially 'true', and the #ifdef-ery
removed, since both KVM_GMEM and KVM_GMEM_SUPPORTS_MMAP are always
selected, no ifs, no buts.

>  
>  #endif /* __ARM64_KVM_HOST_H__ */
> diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> index 713248f240e0..323b46b7c82f 100644
> --- a/arch/arm64/kvm/Kconfig
> +++ b/arch/arm64/kvm/Kconfig
> @@ -37,6 +37,8 @@ menuconfig KVM
>  	select HAVE_KVM_VCPU_RUN_PID_CHANGE
>  	select SCHED_INFO
>  	select GUEST_PERF_EVENTS if PERF_EVENTS
> +	select KVM_GMEM
> +	select KVM_GMEM_SUPPORTS_MMAP
>  	help
>  	  Support hosting virtualized guest machines.
>  
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 8c82df80a835..85559b8a0845 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -2276,6 +2276,13 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>  	if ((new->base_gfn + new->npages) > (kvm_phys_size(&kvm->arch.mmu) >> PAGE_SHIFT))
>  		return -EFAULT;
>  
> +	/*
> +	 * Only support guest_memfd backed memslots with mappable memory, since
> +	 * there aren't any CoCo VMs that support only private memory on arm64.
> +	 */
> +	if (kvm_slot_has_gmem(new) && !kvm_memslot_is_gmem_only(new))
> +		return -EINVAL;
> +
>  	hva = new->userspace_addr;
>  	reg_end = hva + (new->npages << PAGE_SHIFT);
>  

Otherwise,

Reviewed-by: Marc Zyngier <maz@kernel.org>

	M.

-- 
Jazz isn't dead. It just smells funny.