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

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Message-ID: <20220803094827.GA607465@chaop.bj.intel.com>
Date:   Wed, 3 Aug 2022 17:48:27 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.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 Tue, Aug 02, 2022 at 04:38:55PM +0000, Sean Christopherson wrote:
> On Tue, Aug 02, 2022, Sean Christopherson wrote:
> > I think we should avoid UNMAPPABLE even on the KVM side of things for the core
> > memslots functionality and instead be very literal, e.g.
> > 
> > 	KVM_HAS_FD_BASED_MEMSLOTS
> > 	KVM_MEM_FD_VALID
> > 
> > We'll still need KVM_HAS_USER_UNMAPPABLE_MEMORY, but it won't be tied directly to
> > the memslot.  Decoupling the two thingis will require a bit of extra work, but the
> > code impact should be quite small, e.g. explicitly query and propagate
> > MEMFILE_F_USER_INACCESSIBLE to kvm_memory_slot to track if a memslot can be private.
> > And unless I'm missing something, it won't require an additional memslot flag.
> > The biggest oddity (if we don't also add KVM_MEM_PRIVATE) is that KVM would
> > effectively ignore the hva for fd-based memslots for VM types that don't support
> > private memory, i.e. userspace can't opt out of using the fd-based backing, but that
> > doesn't seem like a deal breaker.

I actually love this idea. I don't mind adding extra code for potential
usage other than confidential VMs if we can have a workable solution for
it.

> 
> Hrm, but basing private memory on top of a generic FD_VALID would effectively require
> shared memory to use hva-based memslots for confidential VMs.  That'd yield a very
> weird API, e.g. non-confidential VMs could be backed entirely by fd-based memslots,
> but confidential VMs would be forced to use hva-based memslots.

It would work if we can treat userspace_addr as optional for
KVM_MEM_FD_VALID, e.g. userspace can opt in to decide whether needing
the mappable part or not for a regular VM and we can enforce KVM for
confidential VMs. But the u64 type of userspace_addr doesn't allow us to
express a 'null' value so sounds like we will end up needing another
flag anyway.

In concept, we could have three cofigurations here:
  1. hva-only: without any flag and use userspace_addr;
  2. fd-only:  another new flag is needed and use fd/offset;
  3. hva/fd mixed: both userspace_addr and fd/offset is effective.
     KVM_MEM_PRIVATE is a subset of it for confidential VMs. Not sure
     regular VM also wants this.

There is no direct relationship between unmappable and fd-based since
even fd-based can also be mappable for regular VM?

> 
> Ignore this idea for now.  If there's an actual use case for generic fd-based memory
> then we'll want a separate flag, fd, and offset, i.e. that support could be added
> independent of KVM_MEM_PRIVATE.

If we ignore this idea now (which I'm also fine), do you still think we
need change KVM_MEM_PRIVATE to KVM_MEM_USER_UNMAPPBLE?

Thanks,
Chao