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Date:   Thu, 19 Mar 2020 00:18:24 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To:     "Xing, Cedric" <cedric.xing@intel.com>
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Jethro Beekman <jethro@fortanix.com>,
        Nathaniel McCallum <npmccallum@redhat.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        linux-sgx@vger.kernel.org, akpm@linux-foundation.org,
        dave.hansen@intel.com, Neil Horman <nhorman@redhat.com>,
        "Huang, Haitao" <haitao.huang@intel.com>,
        andriy.shevchenko@linux.intel.com, tglx@linutronix.de,
        "Svahn, Kai" <kai.svahn@intel.com>, bp@alien8.de,
        Josh Triplett <josh@joshtriplett.org>, luto@kernel.org,
        kai.huang@intel.com, David Rientjes <rientjes@google.com>,
        Patrick Uiterwijk <puiterwijk@redhat.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Connor Kuehl <ckuehl@redhat.com>,
        Harald Hoyer <harald@redhat.com>,
        Lily Sturmann <lsturman@redhat.com>
Subject: Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX
 enclave call
Message-ID: <20200318221824.GA55522@linux.intel.com>
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On Thu, Mar 19, 2020 at 12:01:26AM +0200, Jarkko Sakkinen wrote:
> On Mon, Mar 16, 2020 at 04:56:42PM -0700, Xing, Cedric wrote:
> > On 3/16/2020 3:55 PM, Sean Christopherson wrote:
> > > On Mon, Mar 16, 2020 at 02:31:36PM +0100, Jethro Beekman wrote:
> > > > Can someone remind me why we're not passing TCS in RBX but on the stack?
> > > 
> > > I finally remembered why.  It's pulled off the stack and passed into the
> > > exit handler.  I'm pretty sure the vDSO could take it in %rbx and manually
> > > save it on the stack, but I'd rather keep the current behavior so that the
> > > vDSO is callable from C (assuming @leaf is changed to be passed via %rcx).
> > > 
> > The idea is that the caller of this vDSO API is C callable, hence it cannot
> > receive TCS in %rbx anyway. Then it has to either MOV to %rbx or PUSH to
> > stack. Either way the complexity is the same. The vDSO API however has to
> > always save it on stack for exit handler. So receiving it via stack ends up
> > in simplest code.
> 
> It is never C callable anyway given that not following the ABI so
> who cares about being C callable anyway?

A direct quote from the documentation:

"**Important!**  __vdso_sgx_enter_enclave() is **NOT** compliant with
the x86-64 ABI, i.e. cannot be called from standard C code."

/Jarkko