Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

[Nikunj A. Dadhania]

On 15/08/22 18:34, Chao Peng wrote:
> On Fri, Aug 12, 2022 at 02:18:43PM +0530, Nikunj A. Dadhania wrote:
>>
>>
>> On 12/08/22 12:48, Gupta, Pankaj wrote:
>>>
>>>>>>>>
>>>>>>>> However, fallocate() preallocates full guest memory before starting the guest.
>>>>>>>> With this behaviour guest memory is *not* demand pinned. Is there a way to
>>>>>>>> prevent fallocate() from reserving full guest memory?
>>>>>>>
>>>>>>> Isn't the pinning being handled by the corresponding host memory backend with mmu > notifier and architecture support while doing the memory operations e.g page> migration and swapping/reclaim (not supported currently AFAIU). But yes, we need> to allocate entire guest memory with the new flags MEMFILE_F_{UNMOVABLE, UNRECLAIMABLE etc}.
>>>>>>
>>>>>> That is correct, but the question is when does the memory allocated, as these flags are set,
>>>>>> memory is neither moved nor reclaimed. In current scenario, if I start a 32GB guest, all 32GB is
>>>>>> allocated.
>>>>>
>>>>> I guess so if guest memory is private by default.
>>>>>
>>>>> Other option would be to allocate memory as shared by default and
>>>>> handle on demand allocation and RMPUPDATE with page state change event. But still that would be done at guest boot time, IIUC.
>>>>
>>>> Sorry! Don't want to hijack the other thread so replying here.
>>>>
>>>> I thought the question is for SEV SNP. For SEV, maybe the hypercall with the page state information can be used to allocate memory as we use it or something like quota based memory allocation (just thinking).
>>>
>>> But all this would have considerable performance overhead (if by default memory is shared) and used mostly at boot time. 
>>
>>> So, preallocating memory (default memory private) seems better approach for both SEV & SEV SNP with later page management (pinning, reclaim) taken care by host memory backend & architecture together.
>>
>> I am not sure how will pre-allocating memory help, even if guest would not use full memory it will be pre-allocated. Which if I understand correctly is not expected.
> 
> Actually the current version allows you to delay the allocation to a
> later time (e.g. page fault time) if you don't call fallocate() on the
> private fd. fallocate() is necessary in previous versions because we
> treat the existense in the fd as 'private' but in this version we track
> private/shared info in KVM so we don't rely on that fact from memory
> backstores.

Thanks for confirming Chao, in that case we can drop fallocate() from qemu 
in both the case
* Once while creating the memfd private object
* During ram_block_convert_range() for shared->private and vice versa.
 
> Definitely the page will still be pinned once it's allocated, there is
> no way to swap it out for example just with the current code. 

Agree, at present once the page is brought in, page will remain till VM shutdown.

> That kind of support, if desirable, can be extended through MOVABLE flag and some
> other callbacks to let feature-specific code to involve.

Sure, that could be future work.

Regards
Nikunj