Re: [RFC PATCH 30/39] KVM: guest_memfd: Handle folio preparation for guest_memfd mmap

[Patrick Roy]

Hi Ackerley,

On Thu, 2024-10-03 at 22:32 +0100, Ackerley Tng wrote:
> Elliot Berman <quic_eberman@xxxxxxxxxxx> writes:
> 
>> On Tue, Sep 10, 2024 at 11:44:01PM +0000, Ackerley Tng wrote:
>>> Since guest_memfd now supports mmap(), folios have to be prepared
>>> before they are faulted into userspace.
>>>
>>> When memory attributes are switched between shared and private, the
>>> up-to-date flags will be cleared.
>>>
>>> Use the folio's up-to-date flag to indicate being ready for the guest
>>> usage and can be used to mark whether the folio is ready for shared OR
>>> private use.
>>
>> Clearing the up-to-date flag also means that the page gets zero'd out
>> whenever it transitions between shared and private (either direction).
>> pKVM (Android) hypervisor policy can allow in-place conversion between
>> shared/private.
>>
>> I believe the important thing is that sev_gmem_prepare() needs to be
>> called prior to giving page to guest. In my series, I had made a
>> ->prepare_inaccessible() callback where KVM would only do this part.
>> When transitioning to inaccessible, only that callback would be made,
>> besides the bookkeeping. The folio zeroing happens once when allocating
>> the folio if the folio is initially accessible (faultable).
>>
>> From x86 CoCo perspective, I think it also makes sense to not zero
>> the folio when changing faultiblity from private to shared:
>>  - If guest is sharing some data with host, you've wiped the data and
>>    guest has to copy again.
>>  - Or, if SEV/TDX enforces that page is zero'd between transitions,
>>    Linux has duplicated the work that trusted entity has already done.
>>
>> Fuad and I can help add some details for the conversion. Hopefully we
>> can figure out some of the plan at plumbers this week.
> 
> Zeroing the page prevents leaking host data (see function docstring for
> kvm_gmem_prepare_folio() introduced in [1]), so we definitely don't want
> to introduce a kernel data leak bug here.
> 
> In-place conversion does require preservation of data, so for
> conversions, shall we zero depending on VM type?
> 
> + Gunyah: don't zero since ->prepare_inaccessible() is a no-op
> + pKVM: don't zero
> + TDX: don't zero
> + SEV: AMD Architecture Programmers Manual 7.10.6 says there is no
>   automatic encryption and implies no zeroing, hence perform zeroing
> + KVM_X86_SW_PROTECTED_VM: Doesn't have a formal definition so I guess
>   we could require zeroing on transition?

Maybe for KVM_X86_SW_PROTECTED_VM we could make zero-ing configurable
via some CREATE_GUEST_MEMFD flag, instead of forcing one specific
behavior. 

For the "non-CoCo with direct map entries removed" VMs that we at AWS
are going for, we'd like a VM type with host-controlled in-place
conversions which doesn't zero on transitions, so if
KVM_X86_SW_PROTECTED_VM ends up zeroing, we'd need to add another new VM
type for that.

Somewhat related sidenote: For VMs that allow inplace conversions and do
not zero, we do not need to zap the stage-2 mappings on memory attribute
changes, right?

> This way, the uptodate flag means that it has been prepared (as in
> sev_gmem_prepare()), and zeroed if required by VM type.
> 
> Regarding flushing the dcache/tlb in your other question [2], if we
> don't use folio_zero_user(), can we relying on unmapping within core-mm
> to flush after shared use, and unmapping within KVM To flush after
> private use?
> 
> Or should flush_dcache_folio() be explicitly called on kvm_gmem_fault()?
> 
> clear_highpage(), used in the non-hugetlb (original) path, doesn't flush
> the dcache. Was that intended?
> 
>> Thanks,
>> Elliot
>>
>>>
>>> <snip>
> 
> [1] https://lore.kernel.org/all/20240726185157.72821-8-pbonzini@xxxxxxxxxx/
> [2] https://lore.kernel.org/all/diqz34ldszp3.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

Best, 
Patrick