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Date:   Tue, 18 Aug 2020 17:26:25 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To:     Nathaniel McCallum <npmccallum@redhat.com>
Cc:     x86@kernel.org, linux-sgx@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Andy Lutomirski <luto@amacapital.net>,
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        andriy.shevchenko@linux.intel.com, asapek@google.com, bp@alien8.de,
        chenalexchen@google.com, conradparker@google.com,
        cyhanish@google.com, dave.hansen@intel.com,
        "Huang, Haitao" <haitao.huang@intel.com>,
        Josh Triplett <josh@joshtriplett.org>, kai.huang@intel.com,
        "Svahn, Kai" <kai.svahn@intel.com>, kmoy@google.com,
        ludloff@google.com, luto@kernel.org,
        Neil Horman <nhorman@redhat.com>,
        Patrick Uiterwijk <puiterwijk@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Thomas Gleixner <tglx@linutronix.de>, yaozhangx@google.com
Subject: Re: [PATCH v36 21/24] x86/vdso: Implement a vDSO for Intel SGX
 enclave call
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On Thu, Aug 06, 2020 at 10:55:43AM -0400, Nathaniel McCallum wrote:
> In a past revision of this patch, I had requested a void *misc
> parameter that could be passed through vdso_sgx_enter_enclave_t into
> sgx_enclave_exit_handler_t. This request encountered some push back
> and I dropped the issue. However, I'd like to revisit it or something
> similar.
> 
> One way to create a generic interface to SGX is to pass a structure
> that captures the relevant CPU state from the handler so that it can
> be evaluated in C code before reentry. Precedent for this approach can
> be found in struct kvm_run[0]. Currently, however, there is no way to
> pass a pointer to such a structure directly into the handler.
> 
> This can be done implicitly by wrapping the struct
> sgx_enclave_exception in another structure and then using techniques
> like container_of() to find another field. However, this is made more
> difficult by the fact that the sgx_enclave_exit_handler_t is not
> really using the x86_64 sysv calling convention. Therefore, the
> sgx_enclave_exit_handler_t MUST be written in assembly. This also
> implies that we can't use techniques like container_of() and must
> calculate all the offsets manually, which is tedious, error prone and
> fragile.

If instead of having a callback, kernel would run an optional bpf
program, then a state structure could passed by the kernel to the
execution context of the bpf program.

That would also sort out difficulty of writing exit handlers.

/Jarkko