Re: [PATCH v7 06/12] KVM: guest_memfd: add module param for disabling TLB flushing

David Hildenbrand
2025-09-25T20:13:55Z

On 25.09.25 21:59, Dave Hansen wrote:
> On 9/25/25 12:20, David Hildenbrand wrote:
>> On 25.09.25 20:27, Dave Hansen wrote:
>>> On 9/24/25 08:22, Roy, Patrick wrote:
>>>> Add an option to not perform TLB flushes after direct map manipulations.
>>>
>>> I'd really prefer this be left out for now. It's a massive can of worms.
>>> Let's agree on something that works and has well-defined behavior before
>>> we go breaking it on purpose.
>>
>> May I ask what the big concern here is?
>
> It's not a _big_ concern.

Oh, I read "can of worms" and thought there is something seriously
problematic :)

> I just think we want to start on something
> like this as simple, secure, and deterministic as possible.

Yes, I agree. And it should be the default. Less secure would have to be
opt-in and documented thoroughly.

>
> Let's say that with all the unmaps that load_unaligned_zeropad() faults
> start to bite us. It'll take longer to find them if the TLB isn't flushed.
>
> Basically, it'll make the bad things happen sooner rather than later.

Agreed.

--
Cheers

David / dhildenb