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

[Chao Peng]

On Tue, Dec 28, 2021 at 09:48:08PM +0000, Sean Christopherson wrote:
> On Fri, Dec 24, 2021, Chao Peng wrote:
> > On Thu, Dec 23, 2021 at 06:02:33PM +0000, Sean Christopherson wrote:
> > > On Thu, Dec 23, 2021, Chao Peng wrote:
> > > 
> > > 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.
> 
> No, the kernel side maintains an opaque pointer like it does today,

But the opaque pointer will now become memslot, isn't it? That said,
kernel side should maintain a list of opaque pointer (memslot) instead
of one for each fd (inode) since a fd to memslot mapping is 1:M now.

>KVM handles
> reverse engineering the memslot to get the offset and whatever else it needs.
> notify_fallocate() and other callbacks are unchanged, though they probably can
> drop the inode.
> 
> E.g. likely with bad math and handwaving on the overlap detection:
> 
> int kvm_private_fd_fallocate_range(void *owner, pgoff_t start, pgoff_t end)
> {
>       struct kvm_memory_slot *slot = owner;
>       struct kvm_gfn_range gfn_range = {
>               .slot      = slot,
>               .start     = (start - slot->private_offset) >> PAGE_SHIFT,
>               .end       = (end - slot->private_offset) >> PAGE_SHIFT,
>               .may_block = true,
>       };
> 
>       if (!has_overlap(slot, start, end))
>               return 0;
> 
>       gfn_range.end = min(gfn_range.end, slot->base_gfn + slot->npages);
> 
>       kvm_unmap_gfn_range(slot->kvm, &gfn_range);
>       return 0;
> }

I understand this KVM side handling, but again one fd can have multiple
memslots. How shmem decides to notify which memslot from a list of
memslots when it invokes the notify_fallocate()? Or just notify all
the possible memslots then let KVM to check? 

Thanks,
Chao