Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL_GUEST

[David Hildenbrand]

On 19.11.21 14:47, Chao Peng wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> 
> The new seal type provides semantics required for KVM guest private
> memory support. A file descriptor with the seal set is going to be used
> as source of guest memory in confidential computing environments such as
> Intel TDX and AMD SEV.
> 
> F_SEAL_GUEST can only be set on empty memfd. After the seal is set
> userspace cannot read, write or mmap the memfd.
> 
> Userspace is in charge of guest memory lifecycle: it can allocate the
> memory with falloc or punch hole to free memory from the guest.
> 
> The file descriptor passed down to KVM as guest memory backend. KVM
> register itself as the owner of the memfd via memfd_register_guest().
> 
> KVM provides callback that needed to be called on fallocate and punch
> hole.
> 
> memfd_register_guest() returns callbacks that need be used for
> requesting a new page from memfd.
> 

Repeating the feedback I already shared in a private mail thread:


As long as page migration / swapping is not supported, these pages
behave like any longterm pinned pages (e.g., VFIO) or secretmem pages.

1. These pages are not MOVABLE. They must not end up on ZONE_MOVABLE or
MIGRATE_CMA.

That should be easy to handle, you have to adjust the gfp_mask to
	mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
just as mm/secretmem.c:secretmem_file_create() does.

2. These pages behave like mlocked pages and should be accounted as such.

This is probably where the accounting "fun" starts, but maybe it's
easier than I think to handle.

See mm/secretmem.c:secretmem_mmap(), where we account the pages as
VM_LOCKED and will consequently check per-process mlock limits. As we
don't mmap(), the same approach cannot be reused.

See drivers/vfio/vfio_iommu_type1.c:vfio_pin_map_dma() and
vfio_pin_pages_remote() on how to manually account via mm->locked_vm .

But it's a bit hairy because these pages are not actually mapped into
the page tables of the MM, so it might need some thought. Similarly,
these pages actually behave like "pinned" (as in mm->pinned_vm), but we
just don't increase the refcount AFAIR. Again, accounting really is a
bit hairy ...



-- 
Thanks,

David / dhildenb