linux-kernel - Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX enclave call

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Message-ID: <20200315012523.GC208715@linux.intel.com>
Date:   Sun, 15 Mar 2020 03:25:23 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Nathaniel McCallum <npmccallum@...hat.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-sgx@...r.kernel.org, akpm@...ux-foundation.org,
        dave.hansen@...el.com,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Neil Horman <nhorman@...hat.com>,
        "Huang, Haitao" <haitao.huang@...el.com>,
        andriy.shevchenko@...ux.intel.com, tglx@...utronix.de,
        "Svahn, Kai" <kai.svahn@...el.com>, bp@...en8.de,
        Josh Triplett <josh@...htriplett.org>, luto@...nel.org,
        kai.huang@...el.com, David Rientjes <rientjes@...gle.com>,
        cedric.xing@...el.com, Patrick Uiterwijk <puiterwijk@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        Jethro Beekman <jethro@...tanix.com>,
        Connor Kuehl <ckuehl@...hat.com>,
        Harald Hoyer <harald@...hat.com>,
        Lily Sturmann <lsturman@...hat.com>
Subject: Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX
 enclave call

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