Re: [PATCH v10 0/9] KVM: mm: fd-based approach for supporting KVM

[Chao Peng]

> Hi Sean,
> 
> We've rebased the SEV+SNP support onto your updated UPM base support
> tree and things seem to be working okay, but we needed some fixups on
> top of the base support get things working, along with 1 workaround
> for an issue that hasn't been root-caused yet:
> 
>   https://github.com/mdroth/linux/commits/upmv10b-host-snp-v8-wip
> 
>   *stash (upm_base_support): mm: restrictedmem: Kirill's pinning implementation
>   *workaround (use_base_support): mm: restrictedmem: loosen exclusivity check

What I'm seeing is Slot#3 gets added first and then deleted. When it's
gets added, Slot#0 already has the same range bound to restrictedmem so
trigger the exclusive check. This check is exactly the current code for.

>   *fixup (upm_base_support): KVM: use inclusive ranges for restrictedmem binding/unbinding
>   *fixup (upm_base_support): mm: restrictedmem: use inclusive ranges for issuing invalidations

As many kernel APIs treat 'end' as exclusive, I would rather keep using
exclusive 'end' for these APIs(restrictedmem_bind/restrictedmem_unbind
and notifier callbacks) but fix it internally in the restrictedmem. E.g.
all the places where xarray API needs a 'last'/'max' we use 'end - 1'.
See below for the change.

>   *fixup (upm_base_support): KVM: fix restrictedmem GFN range calculations

Subtracting slot->restrictedmem.index for start/end in
restrictedmem_get_gfn_range() is the correct fix.

>   *fixup (upm_base_support): KVM: selftests: CoCo compilation fixes
> 
> We plan to post an updated RFC for v8 soon, but also wanted to share
> the staging tree in case you end up looking at the UPM integration aspects
> before then.
> 
> -Mike

This is the restrictedmem fix to solve 'end' being stored and checked in xarray:

--- a/mm/restrictedmem.c
+++ b/mm/restrictedmem.c
@@ -46,12 +46,12 @@ static long restrictedmem_punch_hole(struct restrictedmem *rm, int mode,
         */
        down_read(&rm->lock);
 
-       xa_for_each_range(&rm->bindings, index, notifier, start, end)
+       xa_for_each_range(&rm->bindings, index, notifier, start, end - 1)
                notifier->ops->invalidate_start(notifier, start, end);
 
        ret = memfd->f_op->fallocate(memfd, mode, offset, len);
 
-       xa_for_each_range(&rm->bindings, index, notifier, start, end)
+       xa_for_each_range(&rm->bindings, index, notifier, start, end - 1)
                notifier->ops->invalidate_end(notifier, start, end);
 
        up_read(&rm->lock);
@@ -224,7 +224,7 @@ static int restricted_error_remove_page(struct address_space *mapping,
                }
                spin_unlock(&inode->i_lock);
 
-               xa_for_each_range(&rm->bindings, index, notifier, start, end)
+               xa_for_each_range(&rm->bindings, index, notifier, start, end - 1)
                        notifier->ops->error(notifier, start, end);
                break;
        }
@@ -301,11 +301,12 @@ int restrictedmem_bind(struct file *file, pgoff_t start, pgoff_t end,
                if (exclusive != rm->exclusive)
                        goto out_unlock;
 
-               if (exclusive && xa_find(&rm->bindings, &start, end, XA_PRESENT))
+               if (exclusive &&
+                   xa_find(&rm->bindings, &start, end - 1, XA_PRESENT))
                        goto out_unlock;
        }
 
-       xa_store_range(&rm->bindings, start, end, notifier, GFP_KERNEL);
+       xa_store_range(&rm->bindings, start, end - 1, notifier, GFP_KERNEL);
        rm->exclusive = exclusive;
        ret = 0;
 out_unlock:
@@ -320,7 +321,7 @@ void restrictedmem_unbind(struct file *file, pgoff_t start, pgoff_t end,
        struct restrictedmem *rm = file->f_mapping->private_data;
 
        down_write(&rm->lock);
-       xa_store_range(&rm->bindings, start, end, NULL, GFP_KERNEL);
+       xa_store_range(&rm->bindings, start, end - 1, NULL, GFP_KERNEL);
        synchronize_rcu();
        up_write(&rm->lock);
 }