Re: [PATCH v3 kvm/queue 05/16] KVM: Maintain ofs_tree for fast memslot lookup by file offset

[Yao Yuan]

On Fri, Dec 24, 2021 at 11:54:18AM +0800, Chao Peng wrote:
> On Thu, Dec 23, 2021 at 06:02:33PM +0000, Sean Christopherson wrote:
> > On Thu, Dec 23, 2021, Chao Peng wrote:
> > > Similar to hva_tree for hva range, maintain interval tree ofs_tree for
> > > offset range of a fd-based memslot so the lookup by offset range can be
> > > faster when memslot count is high.
> >
> > This won't work.  The hva_tree relies on there being exactly one virtual 
> > address
> > space, whereas with private memory, userspace can map multiple files into 
> > the
> > guest at different gfns, but with overlapping offsets.
>
> OK, that's the point.
>
> >
> > I also dislike hijacking __kvm_handle_hva_range() in patch 07.
> >
> > KVM also needs to disallow mapping the same file+offset into multiple gfns, 
> > which
> > I don't see anywhere in this series.
>
> This can be checked against file+offset overlapping with existing slots
> when register a new one.
>
> >
> > In other words, there needs to be a 1:1 gfn:file+offset mapping.  Since 
> > userspace
> > likely wants to allocate a single file for guest private memory and map it 
> > into
> > multiple discontiguous slots, e.g. to skip the PCI hole, the best idea off 
> > the top
> > of my head would be to register the notifier on a per-slot basis, not a 
> > per-VM
> > basis.  It would require a 'struct kvm *' in 'struct kvm_memory_slot', but 
> > that's
> > not a huge deal.
> >
> > That way, KVM's notifier callback already knows the memslot and can compute 
> > overlap
> > between the memslot and the range by reversing the math done by 
> > kvm_memfd_get_pfn().
> > Then, armed with the gfn and slot, invalidation is just a matter of 
> > constructing
> > a struct kvm_gfn_range and invoking kvm_unmap_gfn_range().
>
> KVM is easy but the kernel bits would be difficulty, it has to maintain
> fd+offset to memslot mapping because one fd can have multiple memslots,
> it need decide which memslot needs to be notified.

How about pass "context" of fd (e.g. the gfn/hva start point) when register
the invalidation notifier to fd, then in callback kvm can convert the
offset to absolute hva/gfn with such "context", then do memslot invalidation.

>
> Thanks,
> Chao