linux-kernel - Re: [PATCH v40 19/24] x86/vdso: Implement a vDSO for Intel SGX enclave call

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Date:   Sun, 8 Nov 2020 12:30:27 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jethro Beekman <jethro@...tanix.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        x86@...nel.org, linux-sgx@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        Cedric Xing <cedric.xing@...el.com>, akpm@...ux-foundation.org,
        andriy.shevchenko@...ux.intel.com, asapek@...gle.com, bp@...en8.de,
        chenalexchen@...gle.com, conradparker@...gle.com,
        cyhanish@...gle.com, haitao.huang@...el.com, kai.huang@...el.com,
        kai.svahn@...el.com, kmoy@...gle.com, ludloff@...gle.com,
        luto@...nel.org, nhorman@...hat.com, npmccallum@...hat.com,
        puiterwijk@...hat.com, rientjes@...gle.com, tglx@...utronix.de,
        yaozhangx@...gle.com, mikko.ylinen@...el.com
Subject: Re: [PATCH v40 19/24] x86/vdso: Implement a vDSO for Intel SGX
 enclave call

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.