From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sean Christopherson Subject: Re: [PATCH v17 18/23] platform/x86: Intel SGX driver Date: Fri, 21 Dec 2018 10:28:09 -0800 Message-ID: <20181221182809.GB27371@linux.intel.com> References: <20181217180102.GA12560@linux.intel.com> <20181217183613.GD12491@linux.intel.com> <20181217184333.GA26920@linux.intel.com> <20181217222047.GG12491@linux.intel.com> <20181218154417.GC28326@linux.intel.com> <20181218185349.GC30082@linux.intel.com> <20181219050047.GC14295@linux.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20181219050047.GC14295@linux.intel.com> Sender: linux-kernel-owner@vger.kernel.org To: Jarkko Sakkinen Cc: Andy Lutomirski , Dave Hansen , X86 ML , Platform Driver , linux-sgx@vger.kernel.org, nhorman@redhat.com, npmccallum@redhat.com, "Ayoun, Serge" , shay.katz-zamir@intel.com, Haitao Huang , Andy Shevchenko , Thomas Gleixner , "Svahn, Kai" , mark.shanahan@intel.com, Suresh Siddha , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , Darren Hart , Andy Shevchenko , "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" List-Id: platform-driver-x86.vger.kernel.org On Wed, Dec 19, 2018 at 07:00:47AM +0200, Jarkko Sakkinen wrote: > On Tue, Dec 18, 2018 at 10:53:49AM -0800, Sean Christopherson wrote: > > What if we re-organize the ioctls in such a way that we leave open the > > possibility of allocating raw EPC for KVM via /dev/sgx? I'm not 100% > > positive this approach will work[1], but conceptually it fits well with > > KVM's memory model, e.g. KVM is aware of the GPA<->HVA association but > > generally speaking doesn't know what's physically backing each memory > > region. > > Why would you want to pass EPC through user space to KVM rather than > KVM allocating it through kernel interfaces? Delegating EPC management to userspace fits better with KVM's existing memory ABI. KVM provides a single ioctl(), KVM_SET_USER_MEMORY_REGION[1], that allows userspace to create, move, modify and delete memory regions. Skipping over a lot of details, there are essentially three options for exposing EPC to a KVM guest: 1) Provide a dedicated KVM ioctl() to manage EPC without routing it through KVM_SET_USER_MEMORY_REGION. 2) Add a flag to 'struct kvm_userspace_memory_region' that denotes an EPC memory region and mmap() / allocate EPC in KVM. 3) Provide an ABI to allocate raw EPC and let userspace manage it like any other memory region. Option (1) requires duplicating all of KVM_SET_USER_MEMORY_REGION's functionality unless the ioctl() is severly restricted. Option (2) is an ugly abuse of KVM_SET_USER_MEMORY_REGION since the EPC flag would have completely different semantics than all other usage of KVM_SET_USER_MEMORY_REGION. Thus, option (3). Probably a better question to answer is why provide the ABI through /dev/sgx and not /dev/kvm. IMO /dev/sgx is a more logical way to advertise support to userspace, e.g. userspace can simply check if /dev/sgx (or /dev/sgx/epc) exists vs. probing a KVM capability. Without EPC oversubscription in KVM, /dev/sgx is easily the best fit since the EPC management would reside completely in x86/sgx, i.e. KVM would essentially have zero code related to EPC management. EPC oversubscription complicates things because the architecture forces aspects of VMM oversubscription into the KVM domain, e.g. requires a post-VMXON instruction (ENCLV) and a VM-Exit handler. I still think /dev/sgx is a better fit, my only concern is that the oversubscription code would be even more heinous due to splitting responsibilities. But, Andy's idea of having /dev/sgx/enclave vs. /dev/sgx/epc might help avoid that entirely.