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Date:   Sun, 15 Mar 2020 03:25:23 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To:     Nathaniel McCallum <npmccallum@redhat.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        linux-sgx@vger.kernel.org, akpm@linux-foundation.org,
        dave.hansen@intel.com,
        "Christopherson, Sean J" <sean.j.christopherson@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>,
        cedric.xing@intel.com, Patrick Uiterwijk <puiterwijk@redhat.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Jethro Beekman <jethro@fortanix.com>,
        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: <20200315012523.GC208715@linux.intel.com>
References: <20200303233609.713348-1-jarkko.sakkinen@linux.intel.com>
 <20200303233609.713348-22-jarkko.sakkinen@linux.intel.com>
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On Wed, Mar 11, 2020 at 01:30:07PM -0400, Nathaniel McCallum wrote:
> Currently, the selftest has a wrapper around
> __vdso_sgx_enter_enclave() which preserves all x86-64 ABI callee-saved
> registers (CSRs), though it uses none of them. Then it calls this
> function which uses %rbx but preserves none of the CSRs. Then it jumps
> into an enclave which zeroes all these registers before returning.
> Thus:
> 
>   1. wrapper saves all CSRs
>   2. wrapper repositions stack arguments
>   3. __vdso_sgx_enter_enclave() modifies, but does not save %rbx
>   4. selftest zeros all CSRs
>   5. wrapper loads all CSRs
> 
> I'd like to propose instead that the enclave be responsible for saving
> and restoring CSRs. So instead of the above we have:
>   1. __vdso_sgx_enter_enclave() saves %rbx
>   2. enclave saves CSRs
>   3. enclave loads CSRs
>   4. __vdso_sgx_enter_enclave() loads %rbx
> 
> I know that lots of other stuff happens during enclave transitions,
> but at the very least we could reduce the number of instructions
> through this critical path.

What Jethro said and also that it is a good general principle to cut
down the semantics of any vdso as minimal as possible.

I.e. even if saving RBX would make somehow sense it *can* be left
out without loss in terms of what can be done with the vDSO.

/Jarkko