linux-kernel - Re: [PATCH v3 1/2] x86/sgx: Use sgx_nr_used_pages for EPC page count instead of sgx_nr_free

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Message-ID: <Z__8AHPhZzyhB5A5@kernel.org>
Date: Wed, 16 Apr 2025 21:50:40 +0300
From: Jarkko Sakkinen <jarkko@...nel.org>
To: Elena Reshetova <elena.reshetova@...el.com>
Cc: dave.hansen@...el.com, linux-sgx@...r.kernel.org,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	asit.k.mallick@...el.com, vincent.r.scarlata@...el.com,
	chongc@...gle.com, erdemaktas@...gle.com, vannapurve@...gle.com,
	dionnaglaze@...gle.com, bondarn@...gle.com, scott.raynor@...el.com
Subject: Re: [PATCH v3 1/2] x86/sgx: Use sgx_nr_used_pages for EPC page count
 instead of sgx_nr_free_pages

On Tue, Apr 15, 2025 at 02:51:21PM +0300, Elena Reshetova wrote:
> Note: The serialization for sgx_nr_total_pages is not needed because
> the variable is only updated during the initialization and there's no
> concurrent access.

No. It's

- not a side-note but core part of the rationale.
- the reasoning here is nonsense, or more like it does not exist at all.

sgx_nr_free_pages can be substituted with sgx_nr_used_pages at the sites
where sgx_free_pages was previously used *exactly* because
sgx_reclaimer_init() is called only after sgx_page_cache_init(). This
gives the invariant of it to be constant whenever sgx_alloc_epc_page()
is called.

These type of changes give a proof of the legitimity of the invariant,
which I addressed here.

BR, Jarkko