Re: [PATCH v12 08/18] KVM: guest_memfd: Allow host to map guest_memfd pages

[Ira Weiny <ira.weiny@intel.com>]

Sean Christopherson wrote:
> On Wed, Jun 11, 2025, Fuad Tabba wrote:
> > This patch enables support for shared memory in guest_memfd, including
> 
> Please don't lead with with "This patch", simply state what changes are being
> made as a command.
> 
> > mapping that memory from host userspace.
> 
> > This functionality is gated by the KVM_GMEM_SHARED_MEM Kconfig option,
> > and enabled for a given instance by the GUEST_MEMFD_FLAG_SUPPORT_SHARED
> > flag at creation time.
> 
> Why?  I can see that from the patch.
> 
> This changelog is way, way, waaay too light on details.  Sorry for jumping in at
> the 11th hour, but we've spent what, 2 years working on this? 
> 
> > Reviewed-by: Gavin Shan <gshan@redhat.com>
> > Acked-by: David Hildenbrand <david@redhat.com>
> > Co-developed-by: Ackerley Tng <ackerleytng@google.com>
> > Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> > Signed-off-by: Fuad Tabba <tabba@google.com>
> > ---
> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > index d00b85cb168c..cb19150fd595 100644
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -1570,6 +1570,7 @@ struct kvm_memory_attributes {
> >  #define KVM_MEMORY_ATTRIBUTE_PRIVATE           (1ULL << 3)
> >  
> >  #define KVM_CREATE_GUEST_MEMFD	_IOWR(KVMIO,  0xd4, struct kvm_create_guest_memfd)
> > +#define GUEST_MEMFD_FLAG_SUPPORT_SHARED	(1ULL << 0)
> 
> I find the SUPPORT_SHARED terminology to be super confusing.  I had to dig quite
> deep to undesrtand that "support shared" actually mean "userspace explicitly
> enable sharing on _this_ guest_memfd instance".  E.g. I was surprised to see
> 
> IMO, GUEST_MEMFD_FLAG_SHAREABLE would be more appropriate.  But even that is
> weird to me.  For non-CoCo VMs, there is no concept of shared vs. private.  What's
> novel and notable is that the memory is _mappable_.  Yeah, yeah, pKVM's use case
> is to share memory, but that's a _use case_, not the property of guest_memfd that
> is being controlled by userspace.
> 
> And kvm_gmem_memslot_supports_shared() is even worse.  It's simply that the
> memslot is bound to a mappable guest_memfd instance, it's that the guest_memfd
> instance is the _only_ entry point to the memslot.
> 
> So my vote would be "GUEST_MEMFD_FLAG_MAPPABLE", and then something like

If we are going to change this; FLAG_MAPPABLE is not clear to me either.
The guest can map private memory, right?  I see your point about shared
being overloaded with file shared but it would not be the first time a
term is overloaded.  kvm_slot_has_gmem() does makes a lot of sense.

If it is going to change; how about GUEST_MEMFD_FLAG_USER_MAPPABLE?

Ira

> KVM_MEMSLOT_GUEST_MEMFD_ONLY.  That will make code like this:
> 
> 	if (kvm_slot_has_gmem(slot) &&
> 	    (kvm_gmem_memslot_supports_shared(slot) ||
> 	     kvm_get_memory_attributes(kvm, gfn) & KVM_MEMORY_ATTRIBUTE_PRIVATE)) {
> 		return kvm_gmem_max_mapping_level(slot, gfn, max_level);
> 	}
> 
> much more intutive:
> 
> 	if (kvm_is_memslot_gmem_only(slot) ||
> 	    kvm_get_memory_attributes(kvm, gfn) & KVM_MEMORY_ATTRIBUTE_PRIVATE))
> 		return kvm_gmem_max_mapping_level(slot, gfn, max_level);
> 
> And then have kvm_gmem_mapping_order() do:
> 
> 	WARN_ON_ONCE(!kvm_slot_has_gmem(slot));
> 	return 0;
> 
> >  struct kvm_create_guest_memfd {
> >  	__u64 size;
> > diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> > index 559c93ad90be..e90884f74404 100644
> > --- a/virt/kvm/Kconfig
> > +++ b/virt/kvm/Kconfig
> > @@ -128,3 +128,7 @@ config HAVE_KVM_ARCH_GMEM_PREPARE
> >  config HAVE_KVM_ARCH_GMEM_INVALIDATE
> >         bool
> >         depends on KVM_GMEM
> > +
> > +config KVM_GMEM_SHARED_MEM
> > +       select KVM_GMEM
> > +       bool
> > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > index 6db515833f61..06616b6b493b 100644
> > --- a/virt/kvm/guest_memfd.c
> > +++ b/virt/kvm/guest_memfd.c
> > @@ -312,7 +312,77 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
> >  	return gfn - slot->base_gfn + slot->gmem.pgoff;
> >  }
> >  
> > +static bool kvm_gmem_supports_shared(struct inode *inode)
> > +{
> > +	const u64 flags = (u64)inode->i_private;
> > +
> > +	if (!IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM))
> > +		return false;
> > +
> > +	return flags & GUEST_MEMFD_FLAG_SUPPORT_SHARED;
> > +}
> > +
> > +static vm_fault_t kvm_gmem_fault_shared(struct vm_fault *vmf)
> 
> And to my point about "shared", this is also very confusing, because there are
> zero checks in here about shared vs. private.
> 
> > +{
> > +	struct inode *inode = file_inode(vmf->vma->vm_file);
> > +	struct folio *folio;
> > +	vm_fault_t ret = VM_FAULT_LOCKED;
> > +
> > +	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
> > +		return VM_FAULT_SIGBUS;
> > +
> > +	folio = kvm_gmem_get_folio(inode, vmf->pgoff);
> > +	if (IS_ERR(folio)) {
> > +		int err = PTR_ERR(folio);
> > +
> > +		if (err == -EAGAIN)
> > +			return VM_FAULT_RETRY;
> > +
> > +		return vmf_error(err);
> > +	}
> > +
> > +	if (WARN_ON_ONCE(folio_test_large(folio))) {
> > +		ret = VM_FAULT_SIGBUS;
> > +		goto out_folio;
> > +	}
> > +
> > +	if (!folio_test_uptodate(folio)) {
> > +		clear_highpage(folio_page(folio, 0));
> > +		kvm_gmem_mark_prepared(folio);
> > +	}
> > +
> > +	vmf->page = folio_file_page(folio, vmf->pgoff);
> > +
> > +out_folio:
> > +	if (ret != VM_FAULT_LOCKED) {
> > +		folio_unlock(folio);
> > +		folio_put(folio);
> > +	}
> > +
> > +	return ret;
> > +}
> > +
> > +static const struct vm_operations_struct kvm_gmem_vm_ops = {
> > +	.fault = kvm_gmem_fault_shared,
> > +};
> > +
> > +static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
> > +{
> > +	if (!kvm_gmem_supports_shared(file_inode(file)))
> > +		return -ENODEV;
> > +
> > +	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) !=
> > +	    (VM_SHARED | VM_MAYSHARE)) {
> 
> And the SHARED terminology gets really confusing here, due to colliding with the
> existing notion of SHARED file mappings.
> 
> > +		return -EINVAL;
> > +	}
> > +
> > +	vma->vm_ops = &kvm_gmem_vm_ops;
> > +
> > +	return 0;
> > +}
> > +
> >  static struct file_operations kvm_gmem_fops = {
> > +	.mmap		= kvm_gmem_mmap,
> >  	.open		= generic_file_open,
> >  	.release	= kvm_gmem_release,
> >  	.fallocate	= kvm_gmem_fallocate,
> > @@ -463,6 +533,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
> >  	u64 flags = args->flags;
> >  	u64 valid_flags = 0;
> >  
> > +	if (kvm_arch_supports_gmem_shared_mem(kvm))
> > +		valid_flags |= GUEST_MEMFD_FLAG_SUPPORT_SHARED;
> > +
> >  	if (flags & ~valid_flags)
> >  		return -EINVAL;
> >  
> > -- 
> > 2.50.0.rc0.642.g800a2b2222-goog
> >