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

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 22, 2025, Xiaoyao Li wrote:
> On 7/21/2025 8:22 PM, Xiaoyao Li 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.
> 
> one more findings.
> 
> I tested with QEMU by creating normal (non-private) memory with mmapable
> guest memfd, and enforcily passing the fd of the gmem to struct
> kvm_userspace_memory_region2 when QEMU sets up memory region.
> 
> It hits the kvm_gmem_bind() error since QEMU tries to back different GPA
> region with the same gmem.
> 
> So, the question is do we want to allow the multi-binding for shared-only
> gmem?

Can you elaborate, maybe with code?  I don't think I fully understand the setup.