Re: [PATCH v4 12/12] KVM: Expose KVM_MEM_PRIVATE

[Chao Peng]

On Tue, Jan 25, 2022 at 09:20:39PM +0100, Maciej S. Szmigiero wrote:
> On 18.01.2022 14:21, Chao Peng wrote:
> > KVM_MEM_PRIVATE is not exposed by default but architecture code can turn
> > on it by implementing kvm_arch_private_memory_supported().
> > 
> > Also private memslot cannot be movable and the same file+offset can not
> > be mapped into different GFNs.
> > 
> > Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> > ---
> (..)
> >   static bool kvm_check_memslot_overlap(struct kvm_memslots *slots, int id,
> > -				      gfn_t start, gfn_t end)
> > +				      struct file *file,
> > +				      gfn_t start, gfn_t end,
> > +				      loff_t start_off, loff_t end_off)
> >   {
> >   	struct kvm_memslot_iter iter;
> > +	struct kvm_memory_slot *slot;
> > +	struct inode *inode;
> > +	int bkt;
> >   	kvm_for_each_memslot_in_gfn_range(&iter, slots, start, end) {
> >   		if (iter.slot->id != id)
> >   			return true;
> >   	}
> > +	/* Disallow mapping the same file+offset into multiple gfns. */
> > +	if (file) {
> > +		inode = file_inode(file);
> > +		kvm_for_each_memslot(slot, bkt, slots) {
> > +			if (slot->private_file &&
> > +			     file_inode(slot->private_file) == inode &&
> > +			     !(end_off <= slot->private_offset ||
> > +			       start_off >= slot->private_offset
> > +					     + (slot->npages >> PAGE_SHIFT)))
> > +				return true;
> > +		}
> > +	}
> 
> That's a linear scan of all memslots on each CREATE (and MOVE) operation
> with a fd - we just spent more than a year rewriting similar linear scans
> into more efficient operations in KVM.

In the last version I tried to solve this problem by using interval tree
(just like existing hva_tree), but finally we realized that in one VM we
can have multiple fds with overlapped offsets so that approach is
incorrect. See https://lkml.org/lkml/2021/12/28/480 for the discussion.

So linear scan is used before I can find a better way.

Chao