From: Nathaniel McCallum <npmccallum@redhat.com> To: Sean Christopherson <sean.j.christopherson@intel.com> Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>, linux-kernel@vger.kernel.org, x86@kernel.org, linux-sgx@vger.kernel.org, akpm@linux-foundation.org, dave.hansen@intel.com, Neil Horman <nhorman@redhat.com>, "Huang, Haitao" <haitao.huang@intel.com>, andriy.shevchenko@linux.intel.com, tglx@linutronix.de, "Svahn, Kai" <kai.svahn@intel.com>, bp@alien8.de, Josh Triplett <josh@joshtriplett.org>, luto@kernel.org, kai.huang@intel.com, David Rientjes <rientjes@google.com>, cedric.xing@intel.com, Patrick Uiterwijk <puiterwijk@redhat.com>, Andy Lutomirski <luto@amacapital.net>, Jethro Beekman <jethro@fortanix.com>, Connor Kuehl <ckuehl@redhat.com>, Harald Hoyer <harald@redhat.com>, Lily Sturmann <lsturman@redhat.com> Subject: Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX enclave call Date: Tue, 17 Mar 2020 12:28:58 -0400 Message-ID: <CAOASepNbvproKLFLUvH=+m1xXFeDCWup6O7Pi1py6ENFdarMKg@mail.gmail.com> (raw) In-Reply-To: <20200316225322.GJ24267@linux.intel.com> On Mon, Mar 16, 2020 at 6:53 PM Sean Christopherson <sean.j.christopherson@intel.com> 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 > > > <jarkko.sakkinen@linux.intel.com> wrote: > > > > On Sun, 2020-03-15 at 13:53 -0400, Nathaniel McCallum wrote: > > > > > On Sat, Mar 14, 2020 at 9:25 PM Jarkko Sakkinen > > > > > <jarkko.sakkinen@linux.intel.com> 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. >
next prev parent reply index Thread overview: 109+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-03-03 23:35 [PATCH v28 00/22] Intel SGX foundations Jarkko Sakkinen 2020-03-03 23:35 ` [PATCH v28 01/22] x86/sgx: Update MAINTAINERS Jarkko Sakkinen 2020-03-03 23:35 ` [PATCH v28 02/22] x86/cpufeatures: x86/msr: Add Intel SGX hardware bits Jarkko Sakkinen 2020-03-03 23:35 ` [PATCH v28 03/22] x86/cpufeatures: x86/msr: Intel SGX Launch Control " Jarkko Sakkinen 2020-03-03 23:35 ` [PATCH v28 04/22] x86/mm: x86/sgx: Signal SIGSEGV with PF_SGX Jarkko Sakkinen 2020-03-03 23:35 ` [PATCH v28 05/22] x86/sgx: Add SGX microarchitectural data structures Jarkko Sakkinen 2020-03-03 23:35 ` [PATCH v28 06/22] x86/sgx: Add wrappers for ENCLS leaf functions Jarkko Sakkinen 2020-03-09 21:14 ` Sean Christopherson 2020-03-03 23:35 ` [PATCH v28 07/22] x86/cpu/intel: Detect SGX supprt Jarkko Sakkinen 2020-03-09 21:56 ` Sean Christopherson 2020-03-11 17:03 ` Jarkko Sakkinen 2020-03-03 23:35 ` [PATCH v28 08/22] x86/sgx: Enumerate and track EPC sections Jarkko Sakkinen 2020-03-03 23:35 ` [PATCH v28 09/22] x86/sgx: Add functions to allocate and free EPC pages Jarkko Sakkinen 2020-03-03 23:35 ` [PATCH v28 10/22] mm: Introduce vm_ops->may_mprotect() Jarkko Sakkinen 2020-03-03 23:35 ` [PATCH v28 11/22] x86/sgx: Linux Enclave Driver Jarkko Sakkinen 2020-03-05 17:40 ` Sean Christopherson 2020-03-05 18:24 ` Jethro Beekman 2020-03-05 19:04 ` Sean Christopherson 2020-03-06 19:00 ` Jarkko Sakkinen 2020-03-19 18:22 ` Dr. Greg 2020-03-06 18:58 ` Jarkko Sakkinen 2020-03-03 23:35 ` [PATCH v28 12/22] docs: x86/sgx: Document SGX micro architecture and kernel internals Jarkko Sakkinen 2020-03-03 23:36 ` [PATCH v28 13/22] selftests/x86: Recurse into subdirectories Jarkko Sakkinen 2020-03-03 23:36 ` [PATCH v28 14/22] selftests/x86: Add a selftest for SGX Jarkko Sakkinen 2020-03-04 19:27 ` Nathaniel McCallum 2020-03-05 11:33 ` Jarkko Sakkinen 2020-03-06 15:42 ` Dr. Greg 2020-03-06 19:07 ` Jarkko Sakkinen 2020-03-07 17:42 ` Dr. Greg 2020-03-10 13:08 ` Jarkko Sakkinen 2020-03-11 13:28 ` Jarkko Sakkinen [not found] ` <20200311164047.GG21852@linux.intel.com> 2020-03-13 19:24 ` Jarkko Sakkinen 2020-03-04 19:44 ` Nathaniel McCallum 2020-03-04 19:51 ` Nathaniel McCallum 2020-03-06 5:32 ` Dr. Greg 2020-03-06 19:04 ` Jarkko Sakkinen 2020-03-10 19:29 ` Haitao Huang 2020-03-11 9:13 ` Dr. Greg 2020-03-11 17:15 ` Haitao Huang 2020-03-17 1:07 ` Dr. Greg 2020-03-03 23:36 ` [PATCH v28 15/22] x86/sgx: Add provisioning Jarkko Sakkinen 2020-03-03 23:36 ` [PATCH v28 16/22] x86/sgx: Add a page reclaimer Jarkko Sakkinen 2020-03-05 19:03 ` Sean Christopherson 2020-03-06 18:47 ` Jarkko Sakkinen 2020-03-12 18:38 ` Sean Christopherson 2020-03-15 0:27 ` Jarkko Sakkinen 2020-03-15 1:17 ` Jarkko Sakkinen 2020-03-09 21:16 ` Sean Christopherson 2020-03-03 23:36 ` [PATCH v28 17/22] x86/sgx: ptrace() support for the SGX driver Jarkko Sakkinen 2020-03-03 23:36 ` [PATCH v28 18/22] x86/vdso: Add support for exception fixup in vDSO functions Jarkko Sakkinen 2020-03-03 23:36 ` [PATCH v28 19/22] x86/fault: Add helper function to sanitize error code Jarkko Sakkinen 2020-03-03 23:36 ` [PATCH v28 20/22] x86/traps: Attempt to fixup exceptions in vDSO before signaling Jarkko Sakkinen 2020-03-03 23:36 ` [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX enclave call Jarkko Sakkinen 2020-03-11 17:30 ` Nathaniel McCallum 2020-03-11 17:38 ` Jethro Beekman 2020-03-11 19:15 ` Nathaniel McCallum 2020-03-13 15:48 ` Nathaniel McCallum 2020-03-13 16:46 ` Sean Christopherson 2020-03-13 18:32 ` Nathaniel McCallum 2020-03-13 18:44 ` Sean Christopherson 2020-03-13 20:14 ` Nathaniel McCallum 2020-03-13 22:08 ` Sean Christopherson 2020-03-14 14:10 ` Nathaniel McCallum 2020-03-18 23:40 ` Sean Christopherson 2020-03-19 0:38 ` Xing, Cedric 2020-03-19 1:03 ` Sean Christopherson 2020-03-20 13:55 ` Nathaniel McCallum 2020-03-15 1:25 ` Jarkko Sakkinen 2020-03-15 17:53 ` Nathaniel McCallum 2020-03-16 13:31 ` Jethro Beekman 2020-03-16 13:57 ` Nathaniel McCallum 2020-03-16 13:59 ` Jethro Beekman 2020-03-16 14:03 ` Nathaniel McCallum 2020-03-16 17:17 ` Sean Christopherson 2020-03-16 21:27 ` Jarkko Sakkinen 2020-03-16 21:29 ` Jarkko Sakkinen 2020-03-16 22:55 ` Sean Christopherson 2020-03-16 23:56 ` Xing, Cedric 2020-03-18 22:01 ` Jarkko Sakkinen 2020-03-18 22:18 ` Jarkko Sakkinen 2020-03-16 13:56 ` Jarkko Sakkinen 2020-03-16 14:01 ` Nathaniel McCallum 2020-03-16 21:38 ` Jarkko Sakkinen 2020-03-16 22:53 ` Sean Christopherson 2020-03-16 23:50 ` Xing, Cedric 2020-03-16 23:59 ` Sean Christopherson 2020-03-17 0:18 ` Xing, Cedric 2020-03-17 0:27 ` Sean Christopherson 2020-03-17 16:37 ` Nathaniel McCallum 2020-03-17 16:50 ` Nathaniel McCallum 2020-03-17 21:40 ` Xing, Cedric 2020-03-17 22:09 ` Sean Christopherson 2020-03-17 22:36 ` Xing, Cedric 2020-03-17 23:57 ` Sean Christopherson 2020-03-17 22:23 ` Xing, Cedric 2020-03-18 13:01 ` Nathaniel McCallum 2020-03-20 15:53 ` Nathaniel McCallum 2020-03-17 16:28 ` Nathaniel McCallum [this message] 2020-03-18 22:58 ` Jarkko Sakkinen 2020-03-18 22:39 ` Jarkko Sakkinen 2020-03-11 19:30 ` Nathaniel McCallum 2020-03-13 0:52 ` Sean Christopherson 2020-03-13 16:07 ` Nathaniel McCallum 2020-03-13 16:33 ` Sean Christopherson 2020-03-03 23:36 ` [PATCH v28 22/22] selftests/x86: Add vDSO selftest for SGX Jarkko Sakkinen 2020-03-04 19:24 ` [PATCH v28 00/22] Intel SGX foundations Nathaniel McCallum 2020-03-17 16:00 ` Jordan Hand 2020-03-18 21:56 ` Jarkko Sakkinen 2020-03-19 17:16 ` Dr. Greg
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