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

[Gupta, Pankaj]

> > > > > > > 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.

For SEV I am also not very sure what would be the best way.
There could be a tradeoff between memory pinning and performance.
As I was also thinking about "Async page fault" aspect of guest
in my previous reply. Details needs to be figure out.

Quoting my previous reply here:
"Or maybe later we can think of something like allowing direct page 
fault on host memory access for *SEV* guest as there is no strict 
requirement for memory integrity guarantee and the performance overhead."

Thanks,
Pankaj