From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jarkko Sakkinen Subject: Re: [PATCH v17 18/23] platform/x86: Intel SGX driver Date: Sat, 22 Dec 2018 02:01:45 +0200 Message-ID: <20181222000145.GA8954@linux.intel.com> References: <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> <20181221182809.GB27371@linux.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20181221182809.GB27371@linux.intel.com> Sender: linux-kernel-owner@vger.kernel.org To: Sean Christopherson 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 Fri, Dec 21, 2018 at 10:28:09AM -0800, Sean Christopherson wrote: > > 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). OK, thank you for patience explaining this. > 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. You have to understand that for a KVM non-expert like me it was really important to get the context, which you kindly gave. I have never used KVM's memory management API but now that I know how it works all of this makes perfect sense. This is not a better question but it is definitely a good follow up question :-) I don't really understand you deduction here, however. If SGX was not supported, why couldn't the hypothetical /dev/kvm functionality just return an error? For me it sounds a bit messy that KVM functionality, which is a client to the SGX functionality, places some of its functionality to the SGX core. /Jarkko