Re: [PATCH v4 13/13] RFC: hw/virtio: start virtio-mem guest_memfd regions as shared

[Chenyi Qiang <chenyi.qiang@intel.com>]

On 5/4/2026 8:30 PM, Marc-André Lureau wrote:
> In TDX guests, virtio-mem plug/unplug/re-plug fails because
> kvm_set_phys_mem() unconditionally sets KVM memory attributes to
> PRIVATE for all guest_memfd regions. On re-plug, the PRIVATE->PRIVATE
> transition is a no-op, so KVM doesn't re-AUG pages and the guest's
> TDG.MEM.PAGE.ACCEPT fails.

I think private->private conversion is a no-op success, it will continue
to do KVM_PRE_FAULT_MEMORY in kvm_handle_hc_map_gpa_range() and KVM will AUG pages.

> 
> Implement the "start-shared" approach: virtio-mem memory starts with
> shared KVM attributes. The guest converts shared->private on plug (via
> set_memory_encrypted -> MapGPA + ACCEPT), and back to shared on unplug
> (via set_memory_decrypted). This ensures every plug triggers a real
> SHARED->PRIVATE transition, causing KVM to AUG fresh pages.
> 
> Add RAM_GUEST_MEMFD_START_SHARED flag and set it during virtio-mem
> realize for guest_memfd-backed regions. Use
> ram_block_attributes_state_change() to properly update the attributes
> bitmap through the API. Skip setting PRIVATE in kvm_set_phys_mem()
> when the flag is set. On unplug, explicitly reset KVM attributes to
> shared on the host side to handle the case where the guest skips
> set_memory_decrypted().

If we only want to support unplug the shared memory, should we restrict it to check the attribute
instead of resetting to shared unconditionally?

> 
> See also virtio-comment "[PATCH RFC] virtio-mem: add shared/private memory property details".

Maybe I missed some context, can you provide the link to this RFC patch?