linux-kernel - Re: [PATCH v10 3/9] KVM: Extend the memslot to support fd-based private memory

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Message-ID: <20230106094000.GA2297836@chaop.bj.intel.com>
Date:   Fri, 6 Jan 2023 17:40:00 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        linux-doc@...r.kernel.org, qemu-devel@...gnu.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        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>,
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        Naoya Horiguchi <naoya.horiguchi@....com>,
        Miaohe Lin <linmiaohe@...wei.com>, x86@...nel.org,
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        wei.w.wang@...el.com
Subject: Re: [PATCH v10 3/9] KVM: Extend the memslot to support fd-based
 private memory

On Thu, Jan 05, 2023 at 11:23:01AM +0000, Jarkko Sakkinen wrote:
> On Fri, Dec 02, 2022 at 02:13:41PM +0800, Chao Peng wrote:
> > In memory encryption usage, guest memory may be encrypted with special
> > key and can be accessed only by the guest itself. We call such memory
> > private memory. It's valueless and sometimes can cause problem to allow
> > userspace to access guest private memory. This new KVM memslot extension
> > allows guest private memory being provided through a restrictedmem
> > backed file descriptor(fd) and userspace is restricted to access the
> > bookmarked memory in the fd.
> > 
> > This new extension, indicated by the new flag KVM_MEM_PRIVATE, adds two
> > additional KVM memslot fields restricted_fd/restricted_offset to allow
> > userspace to instruct KVM to provide guest memory through restricted_fd.
> > 'guest_phys_addr' is mapped at the restricted_offset of restricted_fd
> > and the size is 'memory_size'.
> > 
> > The extended memslot can still have the userspace_addr(hva). When use, a
> > single memslot can maintain both private memory through restricted_fd
> > and shared memory through userspace_addr. Whether the private or shared
> > part is visible to guest is maintained by other KVM code.
> > 
> > A restrictedmem_notifier field is also added to the memslot structure to
> > allow the restricted_fd's backing store to notify KVM the memory change,
> > KVM then can invalidate its page table entries or handle memory errors.
> > 
> > Together with the change, a new config HAVE_KVM_RESTRICTED_MEM is added
> > and right now it is selected on X86_64 only.
> > 
> > To make future maintenance easy, internally use a binary compatible
> > alias struct kvm_user_mem_region to handle both the normal and the
> > '_ext' variants.
> 
> Feels bit hacky IMHO, and more like a completely new feature than
> an extension.
> 
> Why not just add a new ioctl? The commit message does not address
> the most essential design here.

Yes, people can always choose to add a new ioctl for this kind of change
and the balance point here is we want to also avoid 'too many ioctls' if
the functionalities are similar.  The '_ext' variant reuses all the
existing fields in the 'normal' variant and most importantly KVM
internally can reuse most of the code. I certainly can add some words in
the commit message to explain this design choice.

Thanks,
Chao
> 
> BR, Jarkko