linux-kernel - RE: [PATCH 2/4] x86/sgx: Change counter sgx_nr_free_pages -> sgx_nr_used

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Message-ID: <DM8PR11MB5750B9CBBB7A51CE3A0F006AE7A42@DM8PR11MB5750.namprd11.prod.outlook.com>
Date: Mon, 24 Mar 2025 12:19:37 +0000
From: "Reshetova, Elena" <elena.reshetova@...el.com>
To: Jarkko Sakkinen <jarkko@...nel.org>
CC: "Hansen, Dave" <dave.hansen@...el.com>, "linux-sgx@...r.kernel.org"
	<linux-sgx@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>, "Mallick,
 Asit K" <asit.k.mallick@...el.com>, "Scarlata, Vincent R"
	<vincent.r.scarlata@...el.com>, "Cai, Chong" <chongc@...gle.com>, "Aktas,
 Erdem" <erdemaktas@...gle.com>, "Annapurve, Vishal" <vannapurve@...gle.com>,
	"dionnaglaze@...gle.com" <dionnaglaze@...gle.com>, "bondarn@...gle.com"
	<bondarn@...gle.com>, "Raynor, Scott" <scott.raynor@...el.com>
Subject: RE: [PATCH 2/4] x86/sgx: Change counter sgx_nr_free_pages ->
 sgx_nr_used_pages

> On Fri, Mar 21, 2025 at 02:34:41PM +0200, 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
> > EPC and a fast way of checking for this, change this variable
> > around to indicate number of used pages instead. The subsequent
> > patch that introduces ENCLS[EUPDATESVN] will take use of this change.
> 
> s/subsequent patch//

Ok

> 
> You should rather express how EUPDATESVN trigger will depend on the
> state of sgx_nr_used_pages and sgx_nr_free_pages.

How about this explanation:

"By counting the # of used pages instead of #of free pages, it allows the
EPC page allocation path execute without a need to take the lock in all
but a single case when the first page is being allocated in EPC. This is
achieved via a fast check in atomic_long_inc_not_zero."

Also, if you think that it is hard to interpret the patch 2/4 without 4/4
I can also squeeze them together and then it becomes right away clear
why the change was done. 

> 
> >
> > No functional changes intended.
> 
> Not really understanding how I should interpret this sentence.

Just as usual: this patch itself doesn’t bring any functional changes
to the way as current SGX code works. I only needed this change to
implement patch 4/4 in more lockless way. 

> 
> The commit message does not mention sgx_nr_used_pages, and neiher it
> makes a case why implementing the feature based on sgx_nr_free_pages is
> not possible.

It is possible to implement it, in fact I did exactly this in the beginning instead,
but as mentioned previously this would have resulted in taking a lock for each
case the page is being allocated. 

Best Regards,
Elena.