linux-kernel - Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL

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Date:   Tue, 23 Nov 2021 22:33:53 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, qemu-devel@...gnu.org,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, john.ji@...el.com, susie.li@...el.com,
        jun.nakajima@...el.com, dave.hansen@...el.com, ak@...ux.intel.com
Subject: Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL_GUEST

On Tue, Nov 23, 2021 at 10:06:02AM +0100, Paolo Bonzini wrote:
> On 11/19/21 16:39, David Hildenbrand wrote:
> > > If qmeu can put all the guest memory in a memfd and not map it, then
> > > I'd also like to see that the IOMMU can use this interface too so we
> > > can have VFIO working in this configuration.
> > 
> > In QEMU we usually want to (and must) be able to access guest memory
> > from user space, with the current design we wouldn't even be able to
> > temporarily mmap it -- which makes sense for encrypted memory only. The
> > corner case really is encrypted memory. So I don't think we'll see a
> > broad use of this feature outside of encrypted VMs in QEMU. I might be
> > wrong, most probably I am:)
> 
> It's not _that_ crazy an idea, but it's going to be some work to teach KVM
> that it has to kmap/kunmap around all memory accesses.
> 
> I think it's great that memfd hooks are usable by more than one subsystem,
> OTOH it's fair that whoever needs it does the work---and VFIO does not need
> it for confidential VMs, yet, so it should be fine for now to have a single
> user.
> 
> On the other hand, as I commented already, the lack of locking in the
> register/unregister functions has to be fixed even with a single user.
> Another thing we can do already is change the guest_ops/guest_mem_ops to
> something like memfd_falloc_notifier_ops/memfd_pfn_ops, and the
> register/unregister functions to memfd_register/unregister_falloc_notifier.

I'm satisified with this naming ;)

> 
> Chao, can you also put this under a new CONFIG such as "bool MEMFD_OPS", and
> select it from KVM?

Yes, reasonable.

> 
> Thanks,
> 
> Paolo