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Subject: Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX
 enclave call
To:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Jethro Beekman <jethro@fortanix.com>
Cc:     Nathaniel McCallum <npmccallum@redhat.com>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.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>
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 <20200315012523.GC208715@linux.intel.com>
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 <20200316225534.GK24267@linux.intel.com>
From:   "Xing, Cedric" <cedric.xing@intel.com>
Message-ID: <7634c48d-a8e2-7366-6f04-06a27f8e5eaf@intel.com>
Date:   Mon, 16 Mar 2020 16:56:42 -0700
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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.