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

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Message-ID: <Z-RecEZhXshD6yma@kernel.org>
Date: Wed, 26 Mar 2025 22:07:12 +0200
From: Jarkko Sakkinen <jarkko@...nel.org>
To: "Reshetova, Elena" <elena.reshetova@...el.com>
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 Mon, Mar 24, 2025 at 12:19:37PM +0000, Reshetova, Elena wrote:
>  
> > 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."

Yep, whole a lot more sense.

> 
> 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. 

Have you benchmarked this (memory barrier vs putting the whole thing
inside spinlock)?

I have doubts that this would even show in margins given how much e.g.,
ELDU takes.

> 
> Best Regards,
> Elena.
> 
> 

BR, Jarkko