Re: [PATCH v14 04/21] KVM: x86: Introduce kvm->arch.supports_gmem

[Ackerley Tng <ackerleytng@google.com>]

Xiaoyao Li <xiaoyao.li@intel.com> writes:

> On 7/15/2025 5:33 PM, Fuad Tabba wrote:
>> Introduce a new boolean member, supports_gmem, to kvm->arch.
>> 
>> Previously, the has_private_mem boolean within kvm->arch was implicitly
>> used to indicate whether guest_memfd was supported for a KVM instance.
>> However, with the broader support for guest_memfd, it's not exclusively
>> for private or confidential memory. Therefore, it's necessary to
>> distinguish between a VM's general guest_memfd capabilities and its
>> support for private memory.
>> 
>> This new supports_gmem member will now explicitly indicate guest_memfd
>> support for a given VM, allowing has_private_mem to represent only
>> support for private memory.
>> 
>> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
>> Reviewed-by: Gavin Shan <gshan@redhat.com>
>> Reviewed-by: Shivank Garg <shivankg@amd.com>
>> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
>> Co-developed-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Fuad Tabba <tabba@google.com>
>
> Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
>
> Btw, it seems that supports_gmem can be enabled for all the types of VM?
>

For now, not really, because supports_gmem allows mmap support, and mmap
support enables KVM_MEMSLOT_GMEM_ONLY, and KVM_MEMSLOT_GMEM_ONLY will
mean that shared faults also get faulted from guest_memfd.

A TDX VM that wants to use guest_memfd for private memory and some other
backing memory for shared memory (let's call this use case "legacy CoCo
VMs") will not work if supports_gmem is just enabled for all types of
VMs, because then shared faults will also go to kvm_gmem_get_pfn().

This will be cleaned up when guest_memfd supports conversion
(guest_memfd stage 2). There, a TDX VM will have .supports_gmem = true.

With guest_memfd stage-2 there will also be a
KVM_CAP_DISABLE_LEGACY_PRIVATE_TRACKING.
KVM_CAP_DISABLE_LEGACY_PRIVATE_TRACKING defaults to false, so for legacy
CoCo VMs, shared faults will go to the other non-guest_memfd memory
source that is configured in userspace_addr as before.

With guest_memfd stage-2, KVM_MEMSLOT_GMEM_ONLY will direct all EPT
faults to kvm_gmem_get_pfn(), but KVM_MEMSLOT_GMEM_ONLY will only be
allowed if KVM_CAP_DISABLE_LEGACY_PRIVATE_TRACKING is true. TDX VMs
wishing to use guest_memfd as the only source of memory for the guest
should set KVM_CAP_DISABLE_LEGACY_PRIVATE_TRACKING to true before
creating the guest_memfd.

> Even without mmap support, allow all the types of VM to create 
> guest_memfd seems not something wrong. It's just that the guest_memfd 
> allocated might not be used, e.g., for KVM_X86_DEFAULT_VM.


































































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