From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 50083C4332E for ; Wed, 18 Mar 2020 22:59:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2720320767 for ; Wed, 18 Mar 2020 22:59:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727277AbgCRW7D (ORCPT ); Wed, 18 Mar 2020 18:59:03 -0400 Received: from mga02.intel.com ([134.134.136.20]:4164 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726647AbgCRW7D (ORCPT ); Wed, 18 Mar 2020 18:59:03 -0400 IronPort-SDR: 9WLFitB2A/CCd9StxPkajMUfvQd1yT7jNwYB3zxIRMalGpC6yLrKnmHLW/YVnz8UI/49+yKWeq BXcA9KdelbxQ== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga002.jf.intel.com ([10.7.209.21]) by orsmga101.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 18 Mar 2020 15:59:02 -0700 IronPort-SDR: Kzst+XzI+DA9X70tno+9tD6RtOdSqyNfKYu4iKARH8e1uGk8kjFAyBcsBsbDzpfY8djoShcw5+ fj3lYTunmHAQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,569,1574150400"; d="scan'208";a="263542576" Received: from mbeldzik-mobl.ger.corp.intel.com (HELO localhost) ([10.252.55.127]) by orsmga002.jf.intel.com with ESMTP; 18 Mar 2020 15:58:51 -0700 Date: Thu, 19 Mar 2020 00:58:50 +0200 From: Jarkko Sakkinen To: Nathaniel McCallum Cc: Sean Christopherson , linux-kernel@vger.kernel.org, x86@kernel.org, linux-sgx@vger.kernel.org, akpm@linux-foundation.org, dave.hansen@intel.com, Neil Horman , "Huang, Haitao" , andriy.shevchenko@linux.intel.com, tglx@linutronix.de, "Svahn, Kai" , bp@alien8.de, Josh Triplett , luto@kernel.org, kai.huang@intel.com, David Rientjes , cedric.xing@intel.com, Patrick Uiterwijk , Andy Lutomirski , Jethro Beekman , Connor Kuehl , Harald Hoyer , Lily Sturmann Subject: Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX enclave call Message-ID: <20200318225850.GD52244@linux.intel.com> References: <20200303233609.713348-1-jarkko.sakkinen@linux.intel.com> <20200303233609.713348-22-jarkko.sakkinen@linux.intel.com> <20200315012523.GC208715@linux.intel.com> <94ce05323c4de721c4a6347223885f2ad9f541af.camel@linux.intel.com> <5dc2ec4bc9433f9beae824759f411c32b45d4b74.camel@linux.intel.com> <20200316225322.GJ24267@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 17, 2020 at 12:28:58PM -0400, Nathaniel McCallum wrote: > On Mon, Mar 16, 2020 at 6:53 PM Sean Christopherson > wrote: > > > > On Mon, Mar 16, 2020 at 11:38:24PM +0200, Jarkko Sakkinen wrote: > > > On Mon, 2020-03-16 at 10:01 -0400, Nathaniel McCallum wrote: > > > > On Mon, Mar 16, 2020 at 9:56 AM Jarkko Sakkinen > > > > wrote: > > > > > On Sun, 2020-03-15 at 13:53 -0400, Nathaniel McCallum wrote: > > > > > > On Sat, Mar 14, 2020 at 9:25 PM Jarkko Sakkinen > > > > > > wrote: > > > > > > > On Wed, Mar 11, 2020 at 01:30:07PM -0400, Nathaniel McCallum wrote: > > > > > > > > Currently, the selftest has a wrapper around > > > > > > > > __vdso_sgx_enter_enclave() which preserves all x86-64 ABI callee-saved > > > > > > > > registers (CSRs), though it uses none of them. Then it calls this > > > > > > > > function which uses %rbx but preserves none of the CSRs. Then it jumps > > > > > > > > into an enclave which zeroes all these registers before returning. > > > > > > > > Thus: > > > > > > > > > > > > > > > > 1. wrapper saves all CSRs > > > > > > > > 2. wrapper repositions stack arguments > > > > > > > > 3. __vdso_sgx_enter_enclave() modifies, but does not save %rbx > > > > > > > > 4. selftest zeros all CSRs > > > > > > > > 5. wrapper loads all CSRs > > > > > > > > > > > > > > > > I'd like to propose instead that the enclave be responsible for saving > > > > > > > > and restoring CSRs. So instead of the above we have: > > > > > > > > 1. __vdso_sgx_enter_enclave() saves %rbx > > > > > > > > 2. enclave saves CSRs > > > > > > > > 3. enclave loads CSRs > > > > > > > > 4. __vdso_sgx_enter_enclave() loads %rbx > > > > > > > > > > > > > > > > I know that lots of other stuff happens during enclave transitions, > > > > > > > > but at the very least we could reduce the number of instructions > > > > > > > > through this critical path. > > > > > > > > > > > > > > What Jethro said and also that it is a good general principle to cut > > > > > > > down the semantics of any vdso as minimal as possible. > > > > > > > > > > > > > > I.e. even if saving RBX would make somehow sense it *can* be left > > > > > > > out without loss in terms of what can be done with the vDSO. > > > > > > > > > > > > Please read the rest of the thread. Sean and I have hammered out some > > > > > > sensible and effective changes. > > > > > > > > > > Have skimmed through that discussion but it comes down how much you get > > > > > by obviously degrading some of the robustness. Complexity of the calling > > > > > pattern is not something that should be emphasized as that is something > > > > > that is anyway hidden inside the runtime. > > > > > > > > My suggestions explicitly maintained robustness, and in fact increased > > > > it. If you think we've lost capability, please speak with specificity > > > > rather than in vague generalities. Under my suggestions we can: > > > > 1. call the vDSO from C > > > > 2. pass context to the handler > > > > 3. have additional stack manipulation options in the handler > > > > > > > > The cost for this is a net 2 additional instructions. No existing > > > > capability is lost. > > > > > > My vague generality in this case is just that the whole design > > > approach so far has been to minimize the amount of wrapping to > > > EENTER. > > > > Yes and no. If we wanted to minimize the amount of wrapping around the > > vDSO's ENCLU then we wouldn't have the exit handler shenanigans in the > > first place. The whole process has been about balancing the wants of each > > use case against the overall quality of the API and code. > > > > > And since this has been kind of agreed by most of the > > > stakeholders doing something against the chosen strategy is > > > something I do hold some resistance. > > > > Up until Nathaniel joined the party, the only stakeholder in terms of the > > exit handler was the Intel SDK. > > I would hope that having additional stakeholders would ease the path > to adoption. > > > There was a general consensus to pass > > registers as-is when there isn't a strong reason to do otherwise. Note > > that Nathaniel has also expressed approval of that approach. > > I still approve that approach. > > > So I think the question that needs to be answered is whether the benefits > > of using %rcx instead of %rax to pass @leaf justify the "pass registers > > as-is" guideline. We've effectively already given this waiver for %rbx, > > as the whole reason why the TCS is passed in on the stack instead of via > > %rbx is so that it can be passed to the exit handler. E.g. the vDSO > > could take the TCS in %rbx and save it on the stack, but we're throwing > > the baby out with the bathwater at that point. > > > > The major benefits being that the vDSO would be callable from C and that > > the kernel could define a legitimate prototype instead of a frankenstein > > prototype that's half assembly and half C. For me, those are significant > > benefits and well worth the extra MOV, PUSH and POP. For some use cases > > it would eliminate the need for an assembly wrapper. For runtimes that > > need an assembly wrapper for whatever reason, it's probably still a win as > > a well designed runtime can avoid register shuffling in the wrapper. And > > if there is a runtime that isn't covered by the above, it's at worst an > > extra MOV. > > > Guys, maybe it is just enough discussing. I see things go in circles at least. Just send a patch against current tree and we'll look into it then? I'm a strong believer of "good enough" well, in everything in life. With a legit patch it is easier to evaluate if what we get is just a different version of good enough, or perhaps we might get some useful value out of it. If you think that you get together something C callable, please *prove* that by also updating the self test. Fair enough? /Jarkko