Re: [PATCH kvmtool v3 0/4] Add guest_memfd support for arm64

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Sun, Jul 12, 2026 at 03:25:32PM +0100, Fuad Tabba wrote:
> Hi folks,
> 
> Changes since v2 [1]:
> - Assert guest_memfd >= 0 in kvm__register_ram_guest_memfd() (Alexandru)
> - Don't split line after = sign (Alexandru)
> - Disable MTE when --guest-memfd is used (Alexandru)
> - Carried Reviewed-bys
> 
> Changes since v1 [2]:
> - Use kvm_userspace_memory_region2 only for guest_memfd slots (Suzuki)
> - Check KVM_CAP_GUEST_MEMFD before KVM_CAP_GUEST_MEMFD_FLAGS (Alexandru)
> - Close guest_memfd fd after unmapping at teardown (Alexandru)
> - Carried Reviewed-bys
> 
> Due to popular demand, i.e., Alexandru asking me about it, I repolished my
> guest_memfd support series. The one I sent years ago was stale and
> over-engineered, so this is a fresh start instead.
> 
> This series adds support for backing guest RAM with guest_memfd on
> arm64, for non-protected VMs under both regular KVM and pKVM.
> 
> The first two patches are standalone fixes in the code this series
> builds on: an uninitialized return value in kvm__for_each_mem_bank(),
> and doubled newlines in die() messages. Patch 3 adds guest_memfd
> plumbing to kvm__register_mem(), using kvm_userspace_memory_region2
> only for guest_memfd-backed slots.
> 
> A new --guest-memfd option creates guest RAM from a guest_memfd with
> mmap support (GUEST_MEMFD_FLAG_MMAP and GUEST_MEMFD_FLAG_INIT_SHARED,
> Linux 6.18+), maps it to provide the userspace mapping, and registers
> it with KVM_SET_USER_MEMORY_REGION2.
> 
> Protected VMs need in-place shared/private conversion of guest_memfd,
> which is still in progress in the kernel [3], so --protected rejects
> the option.
> 
> Based on kvmtool master (ca0ddafa941d).

Looks good to me:

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>

Tested it on an Orion O6 board, by booting a VM with guest_memfd. And also
used it as a base for testing the MTE + guest_memfd patch [1]:

Tested-by: Alexandru Elisei <alexandru.elisei@arm.com>

[1] https://lore.kernel.org/kvmarm/20260714110756.116950-1-alexandru.elisei@arm.com/

Thanks,
Alex

> 
> Cheers,
> /fuad
> 
> [1] https://lore.kernel.org/all/20260708182239.1489297-1-fuad.tabba@linux.dev/
> [2] https://lore.kernel.org/all/20260706083555.302972-1-fuad.tabba@linux.dev/
> [3] https://lore.kernel.org/all/20260618-gmem-inplace-conversion-v8-0-9d2959357853@google.com/
> 
> Fuad Tabba (4):
>   Initialize the return value in kvm__for_each_mem_bank()
>   Remove newline from end of die() aborts
>   Add support for registering guest_memfd-backed memory regions
>   arm64: Add --guest-memfd option to back guest RAM with guest_memfd
> 
>  arm64/include/kvm/kvm-config-arch.h |  5 ++-
>  arm64/kvm.c                         | 44 +++++++++++++++++++++++----
>  hw/cfi_flash.c                      |  2 +-
>  include/kvm/kvm.h                   | 17 +++++++++--
>  include/kvm/util.h                  |  1 +
>  kvm.c                               | 42 ++++++++++++++++++--------
>  util/util.c                         | 47 ++++++++++++++++++++++++++---
>  7 files changed, 131 insertions(+), 27 deletions(-)
> 
> -- 
> 2.39.5
> 
>