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Subject: Re: [RFC PATCH v3 0/4] x86: Add exception fixup for SGX ENCLU
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20181210232449.GA11843@localhost>
Date:   Tue, 11 Dec 2018 07:41:27 -0800
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        linux-sgx@vger.kernel.org,
        Haitao Huang <haitao.huang@linux.intel.com>,
        Jethro Beekman <jethro@fortanix.com>,
        "Dr . Greg Wettstein" <greg@enjellic.com>
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Message-Id: <EEE078E7-7F37-4F74-8874-46E16C1E0A77@amacapital.net>
References: <20181210232141.5425-1-sean.j.christopherson@intel.com> <20181210232449.GA11843@localhost>
To:     Josh Triplett <josh@joshtriplett.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Dec 10, 2018, at 3:24 PM, Josh Triplett <josh@joshtriplett.org> wrote:
>=20
>> On Mon, Dec 10, 2018 at 03:21:37PM -0800, Sean Christopherson wrote:
>> At that point I realized it's a hell of a lot easier to simply provide
>> an IOCTL via /dev/sgx that allows userspace to register a per-process
>> ENCLU exception handler.  At a high level, the basic idea is the same
>> as the vDSO approach: provide a hardcoded fixup handler for ENCLU and
>> attempt to fixup select unhandled exceptions that occurred in user code.
>=20
> So, on the one hand, this is *absolutely* much cleaner than the VDSO
> approach. On the other hand, this is global process state and has some
> of the same problems as a signal handler as a result.

I liked the old version better for this reason and for another reason: while=
 this new one looks very very simple, it still has the hidden complexity tha=
t the magic values written to registers in the event of an exception are ver=
y much Linux specific.

OTOH, the old approach clobbered more regs than needed, but that=E2=80=99s a=
 easy fix.=