Date:   Mon, 22 Nov 2021 11:09:56 -0400
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     David Hildenbrand <david@redhat.com>
Cc:     Chao Peng <chao.p.peng@linux.intel.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, qemu-devel@nongnu.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        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
Subject: Re: [RFC v2 PATCH 01/13] mm/shmem: Introduce F_SEAL_GUEST
Message-ID: <20211122150956.GS876299@ziepe.ca>
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>
 <20211119160023.GI876299@ziepe.ca>
 <4efdccac-245f-eb1f-5b7f-c1044ff0103d@redhat.com>
 <20211122133145.GQ876299@ziepe.ca>
 <56c0dffc-5fc4-c337-3e85-a5c9ce619140@redhat.com>
 <20211122140148.GR876299@ziepe.ca>
 <d2b46b84-8930-4304-2946-4d4a16698b24@redhat.com>
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On Mon, Nov 22, 2021 at 03:57:17PM +0100, David Hildenbrand wrote:
> On 22.11.21 15:01, Jason Gunthorpe wrote:
> > On Mon, Nov 22, 2021 at 02:35:49PM +0100, David Hildenbrand wrote:
> >> On 22.11.21 14:31, Jason Gunthorpe wrote:
> >>> On Mon, Nov 22, 2021 at 10:26:12AM +0100, David Hildenbrand wrote:
> >>>
> >>>> I do wonder if we want to support sharing such memfds between processes
> >>>> in all cases ... we most certainly don't want to be able to share
> >>>> encrypted memory between VMs (I heard that the kernel has to forbid
> >>>> that). It would make sense in the use case you describe, though.
> >>>
> >>> If there is a F_SEAL_XX that blocks every kind of new access, who
> >>> cares if userspace passes the FD around or not?
> >> I was imagining that you actually would want to do some kind of "change
> >> ownership". But yeah, the intended semantics and all use cases we have
> >> in mind are not fully clear to me yet. If it's really "no new access"
> >> (side note: is "access" the right word?) then sure, we can pass the fd
> >> around.
> > 
> > What is "ownership" in a world with kvm and iommu are reading pages
> > out of the same fd?
> 
> In the world of encrypted memory / TDX, KVM somewhat "owns" that memory
> IMHO (for example, only it can migrate or swap out these pages; it's
> might be debatable if the TDX module or KVM actually "own" these pages ).

Sounds like it is a swap provider more than an owner?

Jason