Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 6 Jan 2023 17:40:00 +0800
From:   Chao Peng <chao.p.peng@linux.intel.com>
To:     Jarkko Sakkinen <jarkko@kernel.org>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        linux-doc@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>,
        Arnd Bergmann <arnd@arndb.de>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Miaohe Lin <linmiaohe@huawei.com>, 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>,
        Shuah Khan <shuah@kernel.org>, Mike Rapoport <rppt@kernel.org>,
        Steven Price <steven.price@arm.com>,
        "Maciej S . Szmigiero" <mail@maciej.szmigiero.name>,
        Vlastimil Babka <vbabka@suse.cz>,
        Vishal Annapurve <vannapurve@google.com>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        luto@kernel.org, jun.nakajima@intel.com, dave.hansen@intel.com,
        ak@linux.intel.com, david@redhat.com, aarcange@redhat.com,
        ddutile@redhat.com, dhildenb@redhat.com,
        Quentin Perret <qperret@google.com>, tabba@google.com,
        Michael Roth <michael.roth@amd.com>, mhocko@suse.com,
        wei.w.wang@intel.com
Subject: Re: [PATCH v10 3/9] KVM: Extend the memslot to support fd-based
 private memory
Message-ID: <20230106094000.GA2297836@chaop.bj.intel.com>
Reply-To: Chao Peng <chao.p.peng@linux.intel.com>
References: <20221202061347.1070246-1-chao.p.peng@linux.intel.com>
 <20221202061347.1070246-4-chao.p.peng@linux.intel.com>
 <Y7azFdnnGAdGPqmv@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Y7azFdnnGAdGPqmv@kernel.org>
Precedence: bulk

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