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

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Date:   Thu, 19 Mar 2020 00:39:37 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Nathaniel McCallum <npmccallum@...hat.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-sgx@...r.kernel.org, akpm@...ux-foundation.org,
        dave.hansen@...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 Mon, Mar 16, 2020 at 03:53:22PM -0700, Sean Christopherson wrote:
> Yes and no.   If we wanted to minimize the amount of wrapping around the
> vDSO's ENCLU then we wouldn't have the exit handler shenanigans in the
> first place.  The whole process has been about balancing the wants of each
> use case against the overall quality of the API and code.

Minimizing is not something that happens in a void. Given the user base
for the SDK having the handler was a necessity. Otherwise, we would not
have that handler in the first place.

> Up until Nathaniel joined the party, the only stakeholder in terms of the
> exit handler was the Intel SDK.  There was a general consensus to pass
> registers as-is when there isn't a strong reason to do otherwise.  Note
> that Nathaniel has also expressed approval of that approach.

OK, great.

> The major benefits being that the vDSO would be callable from C and that
> the kernel could define a legitimate prototype instead of a frankenstein
> prototype that's half assembly and half C.  For me, those are significant

I was not aware that there was a plot to make it callable by C.

OK, so right now

A. @leaf =  %eax
B. @tcs = 8(%rsp)
C. @e = 0x10(%rsp)
D. @handler = 0x18(%rsp)

On x86-64 Linux C calling convention means DI/SI/DX/CX type of thing.

So what is the thing that we are referring to C calling convetion in
this email discussion?

> benefits and well worth the extra MOV, PUSH and POP.  For some use cases
> it would eliminate the need for an assembly wrapper.  For runtimes that
> need an assembly wrapper for whatever reason, it's probably still a win as
> a well designed runtime can avoid register shuffling in the wrapper.  And
> if there is a runtime that isn't covered by the above, it's at worst an
> extra MOV.

Is it cool if I rip of the documentation from vsgx_enter_enclave.S and
move it to Documentation/ ? It is nasty to keep and update it where it
is right now. How it is right now, it is destined to rotten.

/Jarkko