linux-kernel - Re: [RFC PATCH v4 5/5] x86/vdso: Add __vdso_sgx_enter_enclave() to wrap SGX enclave transitions

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Date:   Fri, 14 Dec 2018 10:20:39 -0800
From:   Josh Triplett <josh@...htriplett.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Jethro Beekman <jethro@...tanix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "x86@...nel.org" <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Haitao Huang <haitao.huang@...ux.intel.com>,
        "Dr . Greg Wettstein" <greg@...ellic.com>
Subject: Re: [RFC PATCH v4 5/5] x86/vdso: Add __vdso_sgx_enter_enclave() to
 wrap SGX enclave transitions

On Fri, Dec 14, 2018 at 09:03:11AM -0800, Sean Christopherson wrote:
> On Fri, Dec 14, 2018 at 07:38:30AM -0800, Sean Christopherson wrote:
> > On Fri, Dec 14, 2018 at 07:12:04AM -0800, Sean Christopherson wrote:
> > > On Fri, Dec 14, 2018 at 09:55:49AM +0000, Jethro Beekman wrote:
> > > > On 2018-12-14 03:01, Sean Christopherson wrote:
> > > > >+2:	pop	%rbx
> > > > >+	pop	%r12
> > > > >+	pop	%r13
> > > > >+	pop	%r14
> > > > >+	pop	%r15
> > > > >+	pop	%rbp
> > > > >+	ret
> > > > 
> > > > x86-64 ABI requires that you call CLD here (enclave may set it).
> > > 
> > > Ugh.  Technically MXCSR and the x87 CW also need to be preserved.
> > > 
> > > What if rather than treating the enclave as hostile we require it to be
> > > compliant with the x86-64 ABI like any other function?  That would solve
> > > the EFLAGS.DF, MXCSR and x87 issues without adding unnecessary overhead.
> > > And we wouldn't have to save/restore R12-R15.  It'd mean we couldn't use
> > > the stack's red zone to hold @regs and @e, but that's poor form anyways.
> > 
> > Grr, except the processor crushes R12-R15, FCW and MXCSR on asynchronous
> > exits.  But not EFLAGS.DF, that's real helpful.
> 
> I can think of three options that are at least somewhat reasonable:
> 
>   1) Save/restore MXCSR and FCW
> 
>      + 100% compliant with the x86-64 ABI
>      + Callable from any code
>      + Minimal documentation required
>      - Restoring MXCSR/FCW is likely unnecessary 99% of the time
>      - Slow
> 
>   2) Clear EFLAGS.DF but not save/restore MXCSR and FCW
> 
>      + Mostly compliant with the x86-64 ABI
>      + Callable from any code that doesn't use SIMD registers
>      - Need to document deviations from x86-64 ABI
> 
>   3) Require the caller to save/restore everything.
> 
>      + Fast
>      + Userspace can pass all GPRs to the enclave (minus EAX, RBX and RCX)
>      - Completely custom ABI
>      - For all intents and purposes must be called from an assembly wrapper
> 
> 
> Option (3) actually isn't all that awful.  RCX can be used to pass an
> optional pointer to a 'struct sgx_enclave_exception' and we can still
> return standard error codes, e.g. -EFAULT.

Entering and exiting a syscall requires an assembly wrapper, and that
doesn't seem completely unreasonable. It's an easy bit of inline
assembly.