Sender: Paolo Bonzini <paolo.bonzini@gmail.com>
Message-ID: <6de78894-8269-ea3a-b4ee-a5cc4dad827e@redhat.com>
Date:   Tue, 23 Nov 2021 10:06:02 +0100
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To:     David Hildenbrand <david@redhat.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Chao Peng <chao.p.peng@linux.intel.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        qemu-devel@nongnu.org, Jonathan Corbet <corbet@lwn.net>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
        Hugh Dickins <hughd@google.com>,
        Jeff Layton <jlayton@kernel.org>,
        "J . Bruce Fields" <bfields@fieldses.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        luto@kernel.org, john.ji@intel.com, susie.li@intel.com,
        jun.nakajima@intel.com, dave.hansen@intel.com, ak@linux.intel.com
References: <20211119134739.20218-1-chao.p.peng@linux.intel.com>
 <20211119134739.20218-2-chao.p.peng@linux.intel.com>
 <20211119151943.GH876299@ziepe.ca>
 <df11d753-6242-8f7c-cb04-c095f68b41fa@redhat.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL_GUEST
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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.

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

Thanks,

Paolo