Re: [PATCH v3 06/11] KVM: guest_memfd: Add KVM capability to check if guest_memfd is host mappable

[Suzuki K Poulose <suzuki.poulose@arm.com>]

Hi Fuad

On 10/10/2024 09:59, Fuad Tabba wrote:
> Add the KVM capability KVM_CAP_GUEST_MEMFD_MAPPABLE, which is
> true if mapping guest memory is supported by the host.
>
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>   include/uapi/linux/kvm.h | 1 +
>   virt/kvm/kvm_main.c      | 4 ++++
>   2 files changed, 5 insertions(+)
>
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 637efc055145..2c6057bab71c 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -933,6 +933,7 @@ struct kvm_enable_cap {
>   #define KVM_CAP_PRE_FAULT_MEMORY 236
>   #define KVM_CAP_X86_APIC_BUS_CYCLES_NS 237
>   #define KVM_CAP_X86_GUEST_MODE 238
> +#define KVM_CAP_GUEST_MEMFD_MAPPABLE 239
>
>   struct kvm_irq_routing_irqchip {
>       __u32 irqchip;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 77e6412034b9..c2ff09197795 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -5176,6 +5176,10 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
>   #ifdef CONFIG_KVM_PRIVATE_MEM
>       case KVM_CAP_GUEST_MEMFD:
>               return !kvm || kvm_arch_has_private_mem(kvm);
> +#endif
> +#ifdef CONFIG_KVM_GMEM_MAPPABLE
> +     case KVM_CAP_GUEST_MEMFD_MAPPABLE:
> +             return !kvm || kvm_arch_has_private_mem(kvm);

minor nit: Keying this on whether the "kvm" instance has private mem
may not be flexible enough to support other types of CC guest that
may use guestmem, but not "mappable" memory.  e.g. CCA may not
support "mappable", unless we have a way to explicitly pass down
"you can map a shared page from the guest_memfd, but it is not
sharable in place".

We could solve it when we get there, but it might be worth
considering.

Suzuki




>   #endif
>       default:
>               break;

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