Re: [RFC PATCH v1 0/9] KVM: Mapping of guest_memfd at the host and a software protected VM type

David Hildenbrand
2025-01-22T21:42:54Z

On 22.01.25 18:16, Fuad Tabba wrote:
> Hi David,
>
> On Wed, 22 Jan 2025 at 15:41, David Hildenbrand <david@redhat.com> wrote:
>>
>> On 22.01.25 16:35, David Hildenbrand wrote:
>>> On 22.01.25 16:27, Fuad Tabba wrote:
>>>> The purpose of this series is to serve as a potential base for
>>>> restricted mmap() support for guest_memfd [1]. It would allow
>>>> experimentation with what that support would be like, in the safe
>>>> environment of a new VM type used for testing.
>>>>
>>>> This series adds a new VM type for arm64,
>>>> KVM_VM_TYPE_ARM_SW_PROTECTED, analogous to the x86
>>>> KVM_X86_SW_PROTECTED_VM. This type is to serve as a development
>>>> and testing vehicle for Confidential (CoCo) VMs.
>>>>
>>>> Similar to the x86 type, this is currently only for development
>>>> and testing. It's not meant to be used for "real" VMs, and
>>>> especially not in production. The behavior and effective ABI for
>>>> software-protected VMs is unstable.
>>>>
>>>> This series enables mmap() support for guest_memfd specifically
>>>> for the new software-protected VM type, only when explicitly
>>>> enabled in the config.
>>>
>>> Hi!
>>>
>>> IIUC, in this series, there is no "private" vs "shared" distinction,
>>> right? So all pages are mappable, and "conversion" does not exist?
>
> You're right. This is a simplified version of my series that allows
> mmaping of the new KVM_VM_TYPE_ARM_SW_PROTECTED vms to use for
> experimentation.
>
> Cheers,
> /fuad
>
>>
>> Ah, I spot:
>>
>> +#define kvm_arch_private_mem_inplace(kvm) \
>> + (IS_ENABLED(CONFIG_KVM_GMEM_MAPPABLE) && \
>> + ((kvm)->arch.vm_type & KVM_VM_TYPE_ARM_SW_PROTECTED))
>>
>> Which makes me wonder, why don't we need the same way of making sure all
>> references/mappings are gone (+ new page type) when doing the shared ->
>> private conversion? Or is this somewhere in here where I didn't spot it yet?
>
> This is new to this series. The idea, based on a request from Patrick
> Roy, was to have a VM in arm64 we could use to experiment with. Since
> it allows the unconditional mmaping, it's only useful for experiments
> or for non-confidential VMs that want to use guest_memfd.
>
> This series isn't meant to replace the other one, more to supplement
> it and facilitate experimentation while that's going.

Heh, so "kvm_arch_private_mem_inplace" in this series means "no
conversion at all" ? :)

--
Cheers,

David / dhildenb