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Subject: Re: [PATCH v40 19/24] x86/vdso: Implement a vDSO for Intel SGX
 enclave call
To:     Jethro Beekman <jethro@fortanix.com>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        x86@kernel.org, linux-sgx@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Cedric Xing <cedric.xing@intel.com>, akpm@linux-foundation.org,
        andriy.shevchenko@linux.intel.com, asapek@google.com, bp@alien8.de,
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        cyhanish@google.com, haitao.huang@intel.com, kai.huang@intel.com,
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        luto@kernel.org, nhorman@redhat.com, npmccallum@redhat.com,
        puiterwijk@redhat.com, rientjes@google.com, tglx@linutronix.de,
        yaozhangx@google.com, mikko.ylinen@intel.com
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <0b9efcd5-c4e4-f795-820e-f07474289206@intel.com>
Date:   Sun, 8 Nov 2020 12:30:27 -0800
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Precedence: bulk

On 11/8/20 12:20 PM, Jethro Beekman wrote:
>> The vDSO function calling convention uses the standard RDI RSI, RDX, RCX,
>> R8 and R9 registers.  This makes it possible to declare the vDSO as a C
>> prototype, but other than that there is no specific support for SystemV
>> ABI. Things like storing XSAVE are the responsibility of the enclave and
>> the runtime.
> I suppose this may be covered under "no specific support for SystemV
> ABI" but with sgx_enclave_run.user_handler=NULL, R12~R15 *will* get
> clobbered when __vdso_sgx_enter_enclave returns from an SGX AEX. IMO
> this makes the whole "try to be like System V ABI" rather useless,
> but I suppose it doesn't matter too much.

What?

It *says*: "there is no specific support for SystemV ABI."  That's the
point.  It's a tiny *PART* of the ABI, used to "make it possible to
declare the vDSO as a C prototype".

That's far from useless.