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

[David Hildenbrand <david@redhat.com>]

On 27.09.25 09:38, Patrick Roy wrote:
> 
> 
> On Fri, 2025-09-26 at 21:09 +0100, David Hildenbrand wrote:
>> 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.
>>
> 
> For Firecracker, we rely a lot on not preallocating _all_ VM memory, and
> trying to ensure only the actual "working set" of a VM is faulted in (we
> pack a lot more VMs onto a physical host than there is actual physical
> memory available). For VMs that are restored from a snapshot, we know
> pretty well what memory needs to be faulted in (that's where @Nikita's
> write syscall comes in), so there we could try such an optimization. But
> for everything else we very much rely on the on-demand nature of guest
> memory allocation (and hence direct map removal). And even right now,
> the long pole performance-wise are these on-demand faults, so really, we
> don't want them to become even slower :(

Makes sense. I guess even without support for large folios one could 
implement a kind of "fault" around: for example, on access to one addr, 
allocate+prepare all pages in the same 2 M chunk, flushing the tlb only 
once after adjusting all the direct map entries.

> 
> Also, can we really batch multiple TLB flushes as you suggest? Even if
> pages are at consecutive indices in guest_memfd, they're not guaranteed
> to be continguous physically, e.g. we couldn't just coalesce multiple
> TLB flushes into a single TLB flush of a larger range.

Well, you there is the option on just flushing the complete tlb of 
course :) When trying to flush a range you would indeed run into the 
problem of flushing an ever growing range.

> 
> There's probably other things we can try. Backing guest_memfd with
> hugepages would reduce the number TLB flushes by 512x (although not all
> users of Firecracker at Amazon [can] use hugepages).

Right.

> 
> And I do still wonder if it's possible to have "async TLB flushes" where
> we simply don't wait for the IPI (x86 terminology, not sure what the
> mechanism on arm64 is). Looking at
> smp_call_function_many_cond()/invlpgb_kernel_range_flush() on x86, it
> seems so? Although seems like on ARM it's actually just handled by a
> single instruction (TLBI) and not some interprocess communication
> thingy. Maybe there's a variant that's faster / better for this usecase?

Right, some architectures (and IIRC also x86 with some extension) are 
able to flush remote TLBs without IPIs.

Doing a quick search, there seems to be some research on async TLB 
flushing, e.g., [1].

In the context here, I wonder whether an async TLB flush would be 
significantly better than not doing an explicit TLB flush: in both 
cases, it's not really deterministic when the relevant TLB entries will 
vanish: with the async variant it might happen faster on average I guess.


[1] https://cs.yale.edu/homes/abhishek/kumar-taco20.pdf

-- 
Cheers

David / dhildenb