linux-kernel - Re: [PATCH 10/25] x86/sgx: Support enclave page permission changes

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Date:   Thu, 2 Dec 2021 15:48:06 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Reinette Chatre <reinette.chatre@...el.com>,
        dave.hansen@...ux.intel.com, jarkko@...nel.org, tglx@...utronix.de,
        bp@...en8.de, luto@...nel.org, mingo@...hat.com,
        linux-sgx@...r.kernel.org, x86@...nel.org
Cc:     seanjc@...gle.com, kai.huang@...el.com, cathy.zhang@...el.com,
        cedric.xing@...el.com, haitao.huang@...el.com,
        mark.shanahan@...el.com, hpa@...or.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/25] x86/sgx: Support enclave page permission changes

On 12/1/21 11:23 AM, Reinette Chatre wrote:
> + * EPCM permissions can be extended anytime directly from the enclave with
> + * no visibility from the OS. This is accomplished with ENCLU[EMODPE]
> + * run from within enclave. Accessing pages with the new, extended,
> + * permissions requires the OS to update the PTE to handle the subsequent
> + * #PF correctly.

Hi Reinette,

I really dislike the Intel nomenclature here.  I know the Intel docs are
all written around permission "extension", but I find it ambiguous.

I've been looking at these instructions literally for years now and
permission extension to me can mean either:
 1. The set of things you can do is extended
 2. The set of things you can *NOT* do is extended

I much rather prefer nomenclature like:

	EPCM permissions can be relaxed anytime directly from the
	enclave with no visibility from the OS. This is accomplished
	with ENCLU[EMODPE] run from within enclave. Accessing pages with
	the new, relaxed permissions requires the OS to update the PTE
	to handle the subsequent correctly.

"Relax" is less ambiguous.  Relaxing a restriction and relaxing
permissions both mean doing things less strictly.  Extending
restrictions and extending what is allowed are opposites.

Maybe it's just me and I need to get this through my thick skull at some
point.  But, I do think it's OK to improve on the architecture names for
things when they go into the kernel.  The XSAVE XSTATE_BV->xfeatures
rename comes to mind.

Anyway, I'd appreciate if you could keep this in mind and consider
changing it if a future revision is needed if you believe it is more clear.