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: <DM8PR11MB57508706F680790B3FA4CE44E7BD2@DM8PR11MB5750.namprd11.prod.outlook.com>
Date: Wed, 16 Apr 2025 11:50:52 +0000
From: "Reshetova, Elena" <elena.reshetova@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>, "Hansen, Dave" <dave.hansen@...el.com>
CC: "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>, "Scarlata,
 Vincent R" <vincent.r.scarlata@...el.com>, "x86@...nel.org" <x86@...nel.org>,
	"jarkko@...nel.org" <jarkko@...nel.org>, "Annapurve, Vishal"
	<vannapurve@...gle.com>, "Cai, Chong" <chongc@...gle.com>, "Mallick, Asit K"
	<asit.k.mallick@...el.com>, "Aktas, Erdem" <erdemaktas@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"bondarn@...gle.com" <bondarn@...gle.com>, "dionnaglaze@...gle.com"
	<dionnaglaze@...gle.com>, "Raynor, Scott" <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, 2025-04-15 at 14:51 +0300, Elena Reshetova wrote:
> > sgx_nr_free_pages is an atomic that is used to keep track of
> > free EPC pages and detect whenever page reclaiming should start.
> > Since successful execution of ENCLS[EUPDATESVN] requires empty
> 
> The mentioning of ENCLS[EUPDATESVN] is kinda out of blue here.  It's better
> to
> introduce it first like the next patch does.

Thank you, will expand more. 

> 
> > EPC and preferably a fast lockless way of checking for this
> > condition in all code paths where EPC is already used, change the
> > reclaiming code to track the number of used pages via
> > sgx_nr_used_pages instead of sgx_nr_free_pages.
> > For this change to work in the page reclamation code, add a new
> > variable, sgx_nr_total_pages, that will keep track of total
> > number of EPC pages.
> >
> > It would have been possible to implement ENCLS[EUPDATESVN] using
> > existing sgx_nr_free_pages counter and a new sgx_nr_total_pages
> > counter, but it won't be possible to avoid taking a lock *every time*
> > a new EPC page is being allocated. The conversion of sgx_nr_free_pages
> > into sgx_nr_used_pages allows avoiding the lock in all cases except
> > when it is the first EPC page being allocated via a quick
> > atomic_long_inc_not_zero check.
> >
> > 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.
> >
> > Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
> 
> Reviewed-by: Kai Huang <kai.huang@...el.com>

Thank you very much for your review!