Re: [PATCH v15 14/21] KVM: x86: Enable guest_memfd mmap for default VM type

[Fuad Tabba <tabba@google.com>]

Hi Xiaoyao,

On Mon, 21 Jul 2025 at 13:22, Xiaoyao Li <xiaoyao.li@intel.com> wrote:
>
> On 7/18/2025 12:27 AM, Fuad Tabba wrote:
> > +/*
> > + * CoCo VMs with hardware support that use guest_memfd only for backing private
> > + * memory, e.g., TDX, cannot use guest_memfd with userspace mapping enabled.
> > + */
> > +#define kvm_arch_supports_gmem_mmap(kvm)             \
> > +     (IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP) &&   \
> > +      (kvm)->arch.vm_type == KVM_X86_DEFAULT_VM)
>
> I want to share the findings when I do the POC to enable gmem mmap in QEMU.
>
> Actually, QEMU can use gmem with mmap support as the normal memory even
> without passing the gmem fd to kvm_userspace_memory_region2.guest_memfd
> on KVM_SET_USER_MEMORY_REGION2.
>
> Since the gmem is mmapable, QEMU can pass the userspace addr got from
> mmap() on gmem fd to kvm_userspace_memory_region(2).userspace_addr. It
> works well for non-coco VMs on x86.
>
> Then it seems feasible to use gmem with mmap for the shared memory of
> TDX, and an additional gmem without mmap for the private memory. i.e.,
> For struct kvm_userspace_memory_region, the @userspace_addr is passed
> with the uaddr returned from gmem0 with mmap, while @guest_memfd is
> passed with another gmem1 fd without mmap.
>
> However, it fails actually, because the kvm_arch_suports_gmem_mmap()
> returns false for TDX VMs, which means userspace cannot allocate gmem
> with mmap just for shared memory for TDX.
>
> SO my question is do we want to support such case?

Thanks for sharing this. To answer your question, no, we explicitly do
not want to support this feature for TDX, since TDX uses a completely
different paradigm.

Cheers,
/fuad