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

David Hildenbrand
2025-01-23T09:14:27Z

On 23.01.25 10:09, Fuad Tabba wrote:
> On Wed, 22 Jan 2025 at 21:42, David Hildenbrand <david@redhat.com> wrote:
>>
>> 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" ? :)
>
> Yes, just for experimenting with the protected software VM.

Got it, thanks!

--
Cheers,

David / dhildenb