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
