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Subject: Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX
 enclave call
To:     Nathaniel McCallum <npmccallum@redhat.com>
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
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        "Huang, Haitao" <haitao.huang@intel.com>,
        andriy.shevchenko@linux.intel.com, tglx@linutronix.de,
        "Svahn, Kai" <kai.svahn@intel.com>, bp@alien8.de,
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        kai.huang@intel.com, David Rientjes <rientjes@google.com>,
        Patrick Uiterwijk <puiterwijk@redhat.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Jethro Beekman <jethro@fortanix.com>,
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        Harald Hoyer <harald@redhat.com>,
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From:   "Xing, Cedric" <cedric.xing@intel.com>
Message-ID: <afec507a-48cd-a730-586a-b9135cc66315@intel.com>
Date:   Tue, 17 Mar 2020 14:40:34 -0700
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Hi Nathaniel,

I reread your email today and thought I might have misunderstood your 
email earlier. What changes are you asking for exactly? Is that just 
passing @leaf in %ecx rather than in %eax? If so, I wouldn't have any 
problem. I agree with you that the resulted API would then be callable 
from C, even though it wouldn't be able to return back to C due to 
tampered %rbx. But I think the vDSO API can preserve %rbx too, given it 
is used by both EENTER and EEXIT (so is unavailable for parameter 
passing anyway). Alternatively, the C caller can setjmp() to be 
longjmp()'d back from within the exit handler.

-Cedric