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From:   Nathaniel McCallum <npmccallum@redhat.com>
Date:   Wed, 4 Mar 2020 14:51:39 -0500
Message-ID: <CAOASepNLGDGZ=9Rx5Pne5oK7QdQ0deonrSsdUKRsv0fzZtx1Eg@mail.gmail.com>
Subject: Re: [PATCH v28 14/22] selftests/x86: Add a selftest for SGX
To:     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, rientjes@google.com, cedric.xing@intel.com,
        Patrick Uiterwijk <puiterwijk@redhat.com>,
        linux-kselftest@vger.kernel.org
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On Tue, Mar 3, 2020 at 6:39 PM Jarkko Sakkinen
<jarkko.sakkinen@linux.intel.com> wrote:
> diff --git a/tools/testing/selftests/x86/sgx/sgx_call.S b/tools/testing/selftests/x86/sgx/sgx_call.S
> new file mode 100644
> index 000000000000..ca4c7893f9d9
> --- /dev/null
> +++ b/tools/testing/selftests/x86/sgx/sgx_call.S
> @@ -0,0 +1,23 @@
> +/* SPDX-License-Identifier: (GPL-2.0 OR BSD-3-Clause) */
> +/**
> +* Copyright(c) 2016-18 Intel Corporation.
> +*/
> +
> +       .text
> +
> +       .macro ENCLU
> +       .byte 0x0f, 0x01, 0xd7
> +       .endm
> +
> +       .text
> +
> +       .global sgx_call_eenter
> +sgx_call_eenter:
> +       push    %rbx
> +       mov     $0x02, %rax
> +       mov     %rdx, %rbx
> +       lea     sgx_async_exit(%rip), %rcx
> +sgx_async_exit:
> +       ENCLU
> +       pop     %rbx
> +       ret

You need to push and pop all the callee-saved registers here since the
enclave zeros them. This code works today by accident. A future
compiler may emit different register allocation which will cause this
to break.

We might consider making it part of the Linux enclave ABI that the
enclave has to save and restore these registers. This would have a
slight performance advantage in a critical code-path compared to
zeroing and then restoring them. But the VDSO code will need to know
what the expectation is.