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

[Chao Peng]

On Wed, Feb 23, 2022 at 07:32:37PM +0100, Maciej S. Szmigiero wrote:
> On 23.02.2022 13:00, Chao Peng wrote:
> > On Tue, Feb 22, 2022 at 02:16:46AM +0100, Maciej S. Szmigiero wrote:
> > > On 17.02.2022 14:45, Chao Peng wrote:
> > > > 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@linux.intel.com>
> > > > > > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > > > > > ---
> > > > > (..)
> > > > > >     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.
> > > > 
> (..)
> > > > So linear scan is used before I can find a better way.
> > > 
> > > Another option would be to simply not check for overlap at add or move
> > > time, declare such configuration undefined behavior under KVM API and
> > > make sure in MMU notifiers that nothing bad happens to the host kernel
> > > if it turns out somebody actually set up a VM this way (it could be
> > > inefficient in this case, since it's not supposed to ever happen
> > > unless there is a bug somewhere in the userspace part).
> > 
> > Specific to TDX case, SEAMMODULE will fail the overlapping case and then
> > KVM prints a message to the kernel log. It will not cause any other side
> > effect, it does look weird however. Yes warn that in the API document
> > can help to some extent.
> 
> So for the functionality you are adding this code for (TDX) this scan
> isn't necessary and the overlapping case (not supported anyway) is safely
> handled by the hardware (or firmware)?

Yes, it will be handled by the firmware.

> Then I would simply remove the scan and, maybe, add a comment instead
> that the overlap check is done by the hardware.

Sure.

> 
> By the way, if a kernel log message could be triggered by (misbehaving)
> userspace then it should be rate limited (if it isn't already).

Thanks for mention.

Chao
> 
> > Thanks,
> > Chao
> 
> Thanks,
> Maciej