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From: Jethro Beekman <jethro@fortanix.com>
To: Nathaniel McCallum <npmccallum@redhat.com>,
	Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	linux-sgx@vger.kernel.org, akpm@linux-foundation.org,
	dave.hansen@intel.com, "Christopherson,
	Sean J" <sean.j.christopherson@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>,
	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: Mon, 16 Mar 2020 14:31:36 +0100
Message-ID: <7f9f2efe-e9af-44da-6719-040600f5b351@fortanix.com> (raw)
In-Reply-To: <CAOASepP9GeTEqs1DSfPiSm9ER0whj9qwSc46ZiNj_K4dMekOfQ@mail.gmail.com>


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On 2020-03-15 18:53, 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.

I'm not sure they're sensible? By departing from the ENCLU calling convention, both the VDSO and the wrapper become more complicated. The wrapper because now it needs to implement all kinds of logic for different behavior depending on whether the VDSO is or isn't available.

I agree with Jarkko that everything should be kept small and simple. Calling a couple extra instructions is going to have a negligible effect compared to the actual time EENTER/EEXIT take. 

Can someone remind me why we're not passing TCS in RBX but on the stack?

--
Jethro Beekman | Fortanix


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  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 [this message]
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
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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