linux-kernel - Re: [PATCH v7 11/14] KVM: Register/unregister the guest private memory regions

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Date:   Fri, 29 Jul 2022 19:54:09 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chao Peng <chao.p.peng@...ux.intel.com>
Cc:     Wei Wang <wei.w.wang@...ux.intel.com>,
        "Gupta, Pankaj" <pankaj.gupta@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-doc@...r.kernel.org, qemu-devel@...gnu.org,
        linux-kselftest@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        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>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v7 11/14] KVM: Register/unregister the guest private
 memory regions

On Mon, Jul 25, 2022, Chao Peng wrote:
> On Thu, Jul 21, 2022 at 05:58:50PM +0000, Sean Christopherson wrote:
> > On Thu, Jul 21, 2022, Chao Peng wrote:
> > > On Thu, Jul 21, 2022 at 03:34:59PM +0800, Wei Wang wrote:
> > > > 
> > > > 
> > > > On 7/21/22 00:21, Sean Christopherson wrote:
> > > > Maybe you could tag it with cgs for all the confidential guest support
> > > > related stuff: e.g. kvm_vm_ioctl_set_cgs_mem()
> > > > 
> > > > bool is_private = ioctl == KVM_MEMORY_ENCRYPT_REG_REGION;
> > > > ...
> > > > kvm_vm_ioctl_set_cgs_mem(, is_private)
> > > 
> > > If we plan to widely use such abbr. through KVM (e.g. it's well known),
> > > I'm fine.
> > 
> > I'd prefer to stay away from "confidential guest", and away from any VM-scoped
> > name for that matter.  User-unmappable memmory has use cases beyond hiding guest
> > state from the host, e.g. userspace could use inaccessible/unmappable memory to
> > harden itself against unintentional access to guest memory.
> > 
> > > I actually use mem_attr in patch: https://lkml.org/lkml/2022/7/20/610
> > > But I also don't quite like it, it's so generic and sounds say nothing.
> > > 
> > > But I do want a name can cover future usages other than just 
> > > private/shared (pKVM for example may have a third state).
> > 
> > I don't think there can be a third top-level state.  Memory is either private to
> > the guest or it's not.  There can be sub-states, e.g. memory could be selectively
> > shared or encrypted with a different key, in which case we'd need metadata to
> > track that state.
> > 
> > Though that begs the question of whether or not private_fd is the correct
> > terminology.  E.g. if guest memory is backed by a memfd that can't be mapped by
> > userspace (currently F_SEAL_INACCESSIBLE), but something else in the kernel plugs
> > that memory into a device or another VM, then arguably that memory is shared,
> > especially the multi-VM scenario.
> > 
> > For TDX and SNP "private vs. shared" is likely the correct terminology given the
> > current specs, but for generic KVM it's probably better to align with whatever
> > terminology is used for memfd.  "inaccessible_fd" and "user_inaccessible_fd" are
> > a bit odd since the fd itself is accesible.
> > 
> > What about "user_unmappable"?  E.g.
> > 
> >   F_SEAL_USER_UNMAPPABLE, MFD_USER_UNMAPPABLE, KVM_HAS_USER_UNMAPPABLE_MEMORY,
> >   MEMFILE_F_USER_INACCESSIBLE, user_unmappable_fd, etc...
> 
> For KVM I also think user_unmappable looks better than 'private', e.g.
> user_unmappable_fd/KVM_HAS_USER_UNMAPPABLE_MEMORY sounds more
> appropriate names. For memfd however, I don't feel that strong to change
> it from current 'inaccessible' to 'user_unmappable', one of the reason
> is it's not just about unmappable, but actually also inaccessible
> through direct ioctls like read()/write().

Heh, I _knew_ there had to be a catch.  I agree that INACCESSIBLE is better for
memfd.