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

From: Kirill A. Shutemov
Subject: Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL_GUEST
Date: Mon, 22 Nov 2021 16:59:33 +0300

On Fri, Nov 19, 2021 at 02:51:11PM +0100, David Hildenbrand wrote:
> On 19.11.21 14:47, Chao Peng wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> > 
> > 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
> 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.

Okay, fair enough. mapping_set_unevictable() also makes sesne.

> 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 ...

Accounting is fun indeed. Non-mapped mlocked memory is going to be
confusing. Hm...

I will look closer.

 Kirill A. Shutemov

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