Message-ID: <5d11b5f7-3208-4ea8-bbff-f535cf62d576@redhat.com>
Date: Fri, 26 Sep 2025 22:09:07 +0200
From: David Hildenbrand <david@...hat.com>
To: Will Deacon <will@...nel.org>, Patrick Roy <patrick.roy@...ux.dev>
Cc: Dave Hansen <dave.hansen@...el.com>, "Roy, Patrick"
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Subject: Re: [PATCH v7 06/12] KVM: guest_memfd: add module param for disabling
 TLB flushing

On 26.09.25 12:53, Will Deacon wrote:
> On Fri, Sep 26, 2025 at 10:46:15AM +0100, Patrick Roy wrote:
>>
>>
>> On Thu, 2025-09-25 at 21:13 +0100, David Hildenbrand wrote:
>>> 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.
>>
>> Yes, I am definitely happy to have the 100% secure behavior be the
>> default, and the skipping of TLB flushes be an opt-in, with thorough
>> documentation!
>>
>> But I would like to include the "skip tlb flushes" option as part of
>> this patch series straight away, because as I was alluding to in the
>> commit message, with TLB flushes this is not usable for Firecracker for
>> performance reasons :(
> 
> I really don't want that option for arm64. If we're going to bother
> unmapping from the linear map, we should invalidate the TLB.

Reading "TLB flushes result in a up to 40x elongation of page faults in
guest_memfd (scaling with the number of CPU cores), or a 5x elongation
of memory population,", I can understand why one would want that 
optimization :)

@Patrick, couldn't we use fallocate() to preallocate memory and batch 
the TLB flush within such an operation?

That is, we wouldn't flush after each individual direct-map modification 
but after multiple ones part of a single operation like fallocate of a 
larger range.

Likely wouldn't make all use cases happy.

-- 
Cheers

David / dhildenb

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