Re: [PATCH v2 02/12] KVM: arm64: guest_memfd: Check mmap support when preparing memslot

[Steven Price]

On 11/11/2026 15:21, Suzuki K Poulose wrote:
> On 11/11/2026 14:57, Steven Price wrote:
>> Check the guest_memfd backing file when deciding whether an arm64
>> memslot has mappable memory rather than checking whether the slot is
>> gmem only. A public helper function is added to be able to reuse the
>> static kvm_gmem_supports_mmap() function.
>>
> 
> Please could you explain why we need this ? Why not rely on
> kvm_memslot_is_gmem_only(), which already tells
> kvm_gmem_supports_mmap() ? With inplace conversion support gmem supports
> mmap.

Unless I'm missing something it's still possible to create a guest_memfd
that doesn't support mmap() by not passing the GUEST_MEMFD_FLAG_MMAP
flag. So kvm_memslot_is_gmem_only() doesn't actually guarantee that it's
a guest_memfd that supports mmap().

> 
>> Signed-off-by: Steven Price <steven.price@xxxxxxx>
>>
>> ---
>> Changes since v1:
>> - New patch in v2. Check the guest_memfd backing file rather than the
>>    memslot flag when deciding whether arm64 can map the backing memory.
>> ---
>>   arch/arm64/kvm/mmu.c     | 7 ++++---
>>   include/linux/kvm_host.h | 1 +
>>   virt/kvm/guest_memfd.c   | 6 ++++++
>>   3 files changed, 11 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index 1db10d8d6db7..a146f6cde3d0 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -2839,10 +2839,11 @@ int kvm_arch_prepare_memory_region(struct kvm
>> *kvm,
>>           return -EFAULT;
>>         /*
>> -     * Only support guest_memfd backed memslots with mappable memory,
>> since
>> -     * there aren't any CoCo VMs that support only private memory on
>> arm64.
>> +     * Protected VMs currently require guest_memfd backed memslots with
>> +     * mappable memory to support shared mappings.
>>        */
>> -    if (kvm_slot_has_gmem(new) && !kvm_memslot_is_gmem_only(new))
>> +    if (kvm_arch_has_private_mem(kvm) && kvm_slot_has_gmem(new) &&
>> +        !kvm_gmem_slot_supports_mmap(new))

Sashiko has identified I've screwed up this condition though.
kvm_arch_has_private_mem(kvm) shouldn't be in there as that allows a
non-private VM to bind a non-mmappable gmem.

>>           return -EINVAL;
>>         /* guest_memfd is incompatible with MTE. */
>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> index e7fc94e2910a..f65728a713ec 100644
>> --- a/include/linux/kvm_host.h
>> +++ b/include/linux/kvm_host.h
>> @@ -2614,6 +2614,7 @@ static inline bool kvm_is_private_gfn(struct kvm
>> *kvm, gfn_t gfn)
>>     #ifdef CONFIG_KVM_GUEST_MEMFD
>>   bool kvm_gmem_is_private_gfn(struct kvm *kvm, gfn_t gfn);
>> +bool kvm_gmem_slot_supports_mmap(const struct kvm_memory_slot *slot);
>>   bool kvm_arch_supports_gmem_init_shared(struct kvm *kvm);
>>     static inline u64 kvm_gmem_get_supported_flags(struct kvm *kvm)
>> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
>> index 8c97a69b16dc..3dd8d227bebb 100644
>> --- a/virt/kvm/guest_memfd.c
>> +++ b/virt/kvm/guest_memfd.c
>> @@ -442,6 +442,12 @@ static bool kvm_gmem_supports_mmap(struct inode
>> *inode)
>>       return GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_MMAP;
>>   }
>>   +bool kvm_gmem_slot_supports_mmap(const struct kvm_memory_slot *slot)
>> +{
>> +    /* The guest_memfd file is pinned while preparing a new memslot. */
> 
> We should gate this on kvm_slot_has_gmem() and how do we ensure that the
> call comes after it is prepared ?

I'm not sure what you mean by "gate this on kvm_slot_has_gmem()" - yes
this function only makes sense on a gmem slot - the user above checks
that first. I'm not sure what value the function should call if you call
it on a non-gmem slot?

Thanks,
Steve

>> +    return kvm_gmem_supports_mmap(file_inode(slot->gmem.file));
>> +}
>> +
>>   static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
>>   {
>>       struct inode *inode = file_inode(vmf->vma->vm_file);
> 
> 
> Cheers
> Suzuki