Re: [RFC PATCH v12 14/33] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory
From: Sean Christopherson
Date: Wed Sep 20 2023 - 19:44:58 EST
On Mon, Sep 18, 2023, Michael Roth wrote:
> > +static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len)
> > +{
> > + struct list_head *gmem_list = &inode->i_mapping->private_list;
> > + pgoff_t start = offset >> PAGE_SHIFT;
> > + pgoff_t end = (offset + len) >> PAGE_SHIFT;
> > + struct kvm_gmem *gmem;
> > +
> > + /*
> > + * Bindings must stable across invalidation to ensure the start+end
> > + * are balanced.
> > + */
> > + filemap_invalidate_lock(inode->i_mapping);
> > +
> > + list_for_each_entry(gmem, gmem_list, entry) {
> > + kvm_gmem_invalidate_begin(gmem, start, end);
>
> In v11 we used to call truncate_inode_pages_range() here to drop filemap's
> reference on the folio. AFAICT the folios are only getting free'd upon
> guest shutdown without this. Was this on purpose?
Nope, I just spotted this too. And then after scratching my head for a few minutes,
wondering if I was having an -ENOCOFFEE moment, I finally read your mail. *sigh*
Looking at my reflog history, I'm pretty sure I deleted the wrong line when
removing the truncation from kvm_gmem_error_page().
> > + kvm_gmem_invalidate_end(gmem, start, end);
> > + }
> > +
> > + filemap_invalidate_unlock(inode->i_mapping);
> > +
> > + return 0;
> > +}
> > +
> > +static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len)
> > +{
> > + struct address_space *mapping = inode->i_mapping;
> > + pgoff_t start, index, end;
> > + int r;
> > +
> > + /* Dedicated guest is immutable by default. */
> > + if (offset + len > i_size_read(inode))
> > + return -EINVAL;
> > +
> > + filemap_invalidate_lock_shared(mapping);
>
> We take the filemap lock here, but not for
> kvm_gmem_get_pfn()->kvm_gmem_get_folio(). Is it needed there as well?
No, we specifically do not want to take a rwsem when faulting in guest memory.
Callers of kvm_gmem_get_pfn() *must* guard against concurrent invalidations via
mmu_invalidate_seq and friends.
> > + /*
> > + * For simplicity, require the offset into the file and the size of the
> > + * memslot to be aligned to the largest possible page size used to back
> > + * the file (same as the size of the file itself).
> > + */
> > + if (!kvm_gmem_is_valid_size(offset, flags) ||
> > + !kvm_gmem_is_valid_size(size, flags))
> > + goto err;
>
> I needed to relax this check for SNP. KVM_GUEST_MEMFD_ALLOW_HUGEPAGE
> applies to entire gmem inode, so it makes sense for userspace to enable
> hugepages if start/end are hugepage-aligned, but QEMU will do things
> like map overlapping regions for ROMs and other things on top of the
> GPA range that the gmem inode was originally allocated for. For
> instance:
>
> 692500@1689108688.696338:kvm_set_user_memory AddrSpace#0 Slot#0 flags=0x4 gpa=0x0 size=0x80000000 ua=0x7fbf5be00000 ret=0 restricted_fd=19 restricted_offset=0x0
> 692500@1689108688.699802:kvm_set_user_memory AddrSpace#0 Slot#1 flags=0x4 gpa=0x100000000 size=0x380000000 ua=0x7fbfdbe00000 ret=0 restricted_fd=19 restricted_offset=0x80000000
> 692500@1689108688.795412:kvm_set_user_memory AddrSpace#0 Slot#0 flags=0x0 gpa=0x0 size=0x0 ua=0x7fbf5be00000 ret=0 restricted_fd=19 restricted_offset=0x0
> 692500@1689108688.795550:kvm_set_user_memory AddrSpace#0 Slot#0 flags=0x4 gpa=0x0 size=0xc0000 ua=0x7fbf5be00000 ret=0 restricted_fd=19 restricted_offset=0x0
> 692500@1689108688.796227:kvm_set_user_memory AddrSpace#0 Slot#6 flags=0x4 gpa=0x100000 size=0x7ff00000 ua=0x7fbf5bf00000 ret=0 restricted_fd=19 restricted_offset=0x100000
>
> Because of that the KVM_SET_USER_MEMORY_REGIONs for non-THP-aligned GPAs
> will fail. Maybe instead it should be allowed, and kvm_gmem_get_folio()
> should handle the alignment checks on a case-by-case and simply force 4k
> for offsets corresponding to unaligned bindings?
Yeah, I wanted to keep the code simple, but disallowing small bindings/memslots
is probably going to be a deal-breaker. Even though I'm skeptical that QEMU
_needs_ to play these games for SNP guests, not playing nice will make it all
but impossible to use guest_memfd for regular VMs.
And the code isn't really any more complex, so long as we punt on allowing
hugepages on interior sub-ranges.
Compile-tested only, but this?
---
virt/kvm/guest_mem.c | 54 ++++++++++++++++++++++----------------------
1 file changed, 27 insertions(+), 27 deletions(-)
diff --git a/virt/kvm/guest_mem.c b/virt/kvm/guest_mem.c
index a819367434e9..dc12e38211df 100644
--- a/virt/kvm/guest_mem.c
+++ b/virt/kvm/guest_mem.c
@@ -426,20 +426,6 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags,
return err;
}
-static bool kvm_gmem_is_valid_size(loff_t size, u64 flags)
-{
- if (size < 0 || !PAGE_ALIGNED(size))
- return false;
-
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
- if ((flags & KVM_GUEST_MEMFD_ALLOW_HUGEPAGE) &&
- !IS_ALIGNED(size, HPAGE_PMD_SIZE))
- return false;
-#endif
-
- return true;
-}
-
int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
{
loff_t size = args->size;
@@ -452,9 +438,15 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
if (flags & ~valid_flags)
return -EINVAL;
- if (!kvm_gmem_is_valid_size(size, flags))
+ if (size < 0 || !PAGE_ALIGNED(size))
return -EINVAL;
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+ if ((flags & KVM_GUEST_MEMFD_ALLOW_HUGEPAGE) &&
+ !IS_ALIGNED(size, HPAGE_PMD_SIZE))
+ return false;
+#endif
+
return __kvm_gmem_create(kvm, size, flags, kvm_gmem_mnt);
}
@@ -462,7 +454,7 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
unsigned int fd, loff_t offset)
{
loff_t size = slot->npages << PAGE_SHIFT;
- unsigned long start, end, flags;
+ unsigned long start, end;
struct kvm_gmem *gmem;
struct inode *inode;
struct file *file;
@@ -481,16 +473,9 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
goto err;
inode = file_inode(file);
- flags = (unsigned long)inode->i_private;
- /*
- * For simplicity, require the offset into the file and the size of the
- * memslot to be aligned to the largest possible page size used to back
- * the file (same as the size of the file itself).
- */
- if (!kvm_gmem_is_valid_size(offset, flags) ||
- !kvm_gmem_is_valid_size(size, flags))
- goto err;
+ if (offset < 0 || !PAGE_ALIGNED(offset))
+ return -EINVAL;
if (offset + size > i_size_read(inode))
goto err;
@@ -591,8 +576,23 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
page = folio_file_page(folio, index);
*pfn = page_to_pfn(page);
- if (max_order)
- *max_order = compound_order(compound_head(page));
+ if (!max_order)
+ goto success;
+
+ *max_order = compound_order(compound_head(page));
+ if (!*max_order)
+ goto success;
+
+ /*
+ * For simplicity, allow mapping a hugepage if and only if the entire
+ * binding is compatible, i.e. don't bother supporting mapping interior
+ * sub-ranges with hugepages (unless userspace comes up with a *really*
+ * strong use case for needing hugepages within unaligned bindings).
+ */
+ if (!IS_ALIGNED(slot->gmem.pgoff, 1ull << *max_order) ||
+ !IS_ALIGNED(slot->npages, 1ull << *max_order))
+ *max_order = 0;
+success:
r = 0;
out_unlock:
base-commit: bc1a54ee393e0574ea422525cf0b2f1e768e38c5
--