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Date:   Fri, 14 Dec 2018 07:04:37 -0800
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Jethro Beekman <jethro@fortanix.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "x86@kernel.org" <x86@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Haitao Huang <haitao.huang@linux.intel.com>,
        "linux-sgx@vger.kernel.org" <linux-sgx@vger.kernel.org>,
        "Dr. Greg" <greg@enjellic.com>
Subject: Re: [RFC PATCH v2 4/4] x86/vdso: Add __vdso_sgx_enter_enclave() to
 wrap SGX enclave transitions
Message-ID: <20181214150436.GA23255@linux.intel.com>
References: <20181206221922.31012-1-sean.j.christopherson@intel.com>
 <20181206221922.31012-5-sean.j.christopherson@intel.com>
 <f2b7df05-20ef-ac16-fc99-258c39d84666@fortanix.com>
 <59c989a1-e699-9665-780f-6dd263f41ce4@intel.com>
 <f595c046-682c-0d4a-ce68-44d4634cedf2@fortanix.com>
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On Sat, Dec 08, 2018 at 08:15:38AM +0000, Jethro Beekman wrote:
> On 2018-12-08 00:14, Dave Hansen wrote:
> >On 12/7/18 10:15 AM, Jethro Beekman wrote:
> >>This is not sufficient to support the Fortanix SGX ABI calling
> >>convention, which was designed to be mostly compatible with the SysV
> >>64-bit calling convention. The following registers need to be passed in
> >>to an enclave from userspace: RDI, RSI, RDX, R8, R9, R10. The following
> >>registers need to be passed out from an enclave to userspace: RDI, RSI,
> >>RDX, R8, R9.
> >
> >Are you asking nicely to change the new Linux ABI to be consistent with
> >your existing ABI?  Or, are you saying that the new ABI *must* be
> >compatible with this previous out-of-tree implementation?
> 
> What's being discussed here is one of the alternatives for SGX fault
> handling, meant to improve the current status quo of having to use a signal
> handler.
> 
> I'm merely providing a data point that the currently proposed solution is
> not sufficient to support current use of the (ring 3) ENCLU instruction. You
> might find this useful in determining whether proposed kernel features will
> actually be used by users, and in further developing this solution or other
> solutions to the fault handling issue.
> 
> If going with the vDSO solution, I think something with semantics closer to
> the actual instruction would be preferred, like the following:
> 
> notrace __attribute__((naked)) long __vdso_sgx_enclu_with_aep()
> {
> 	asm volatile(
> 		"	lea	2f(%%rip), %%rcx\n"
> 		"1:	enclu\n"
> 		"2:     ret\n"
> 		".pushsection .fixup, \"ax\" \n"
> 		"3:	jmp 2b\n"
> 		".popsection\n"
> 		_ASM_VDSO_EXTABLE_HANDLE(1b, 3b)
> 		:::
> 	);
> }

Part of me likes this idea, but it's a documentation nightmare since
it's a completely customer register ABI.  And the caller's exception
handling gets a bit weird since RAX implicitly defines whether or not
an exception occurred.  I also think there's value in making the vDSO
function callable from standard C.