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Date: Fri, 31 Aug 2018 14:22:45 +0300 From: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com> To: Sean Christopherson <sean.j.christopherson@...el.com> Cc: Dave Hansen <dave.hansen@...el.com>, x86@...nel.org, platform-driver-x86@...r.kernel.org, nhorman@...hat.com, npmccallum@...hat.com, linux-sgx@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, Suresh Siddha <suresh.b.siddha@...el.com>, Serge Ayoun <serge.ayoun@...el.com>, "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v13 09/13] x86/sgx: Enclave Page Cache (EPC) memory manager On Tue, Aug 28, 2018 at 02:22:44PM -0700, Sean Christopherson wrote: > On Tue, Aug 28, 2018 at 07:07:33AM -0700, Dave Hansen wrote: > > On 08/28/2018 01:35 AM, Jarkko Sakkinen wrote: > > > On Mon, Aug 27, 2018 at 02:15:34PM -0700, Dave Hansen wrote: > > >> On 08/27/2018 11:53 AM, Jarkko Sakkinen wrote: > > >>> +struct sgx_epc_page_ops { > > >>> + bool (*get)(struct sgx_epc_page *epc_page); > > >>> + void (*put)(struct sgx_epc_page *epc_page); > > >>> + bool (*reclaim)(struct sgx_epc_page *epc_page); > > >>> + void (*block)(struct sgx_epc_page *epc_page); > > >>> + void (*write)(struct sgx_epc_page *epc_page); > > >>> +}; > > >> Why do we need a fancy, slow (retpoline'd) set of function pointers when > > >> we only have one user of these (the SGX driver)? > > > KVM has its own implementation for these operations. > > > > That belongs in the changelog. > > > > Also, where is the implementation? How can we assess this code that was > > built to create an abstraction without both of the users? > > I can provide an early preview of the KVM reclaim code, but honestly > I think that would do more harm than good. The VMX architecture for > EPC reclaim is complex, even for SGX standards. Opening that can of > worms would likely derail this discussion. That being said, this > abstraction isn't exactly what KVM will need, but it's pretty close > and gives us something to build on. > > Regardless, this layer of indirection is justifiable even with a > single implementation. Reclaiming an EPC page is not a simple matter > of copying the data somewhere else and marking the page not present. > Actual eviction requires a reference to the Secure Enclave Control > Structure (SECS) of the enclave that owns the page. Software needs > to ensure it doesn't violate the hardware-enforced access rules, e.g. > most reclaim activites need to be done under a per-enclave lock. > Not all pages have the same reclaim rules, e.g. an SECS can only > be reclaimed after all its child pages have reclaimed, a VA page > doesn't need to be blocked, etc... And the list goes on... To simplify a bit what Sean said about the key difference to a standard page is that in SGX a page is part of a hierarchical structure. EPC pages have hardware enforced dependencies to each other. Examples: 1. You cannot delete or swap SECS before its children have been deleted or swapped. You get SGX_CHILD_PRESENT erro from EREMOVE. 2. In order to swap or fault a page you need to have an EPC page that holds a version number for the page you want to swap. These are called Version Array (VA) pages. /Jarkko
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