linux-kernel - RE: [PATCH v14 5/5] x86/sgx: Enable automatic SVN updates for SGX enclaves

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Message-ID: <DM8PR11MB5750EADDEFCA1160E6E75B0AE734A@DM8PR11MB5750.namprd11.prod.outlook.com>
Date: Fri, 15 Aug 2025 06:59:23 +0000
From: "Reshetova, Elena" <elena.reshetova@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>, "Hansen, Dave" <dave.hansen@...el.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "mingo@...nel.org"
	<mingo@...nel.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>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Mallick, Asit K" <asit.k.mallick@...el.com>,
	"Aktas, Erdem" <erdemaktas@...gle.com>, "Cai, Chong" <chongc@...gle.com>,
	"Bondarevska, Nataliia" <bondarn@...gle.com>, "linux-sgx@...r.kernel.org"
	<linux-sgx@...r.kernel.org>, "Raynor, Scott" <scott.raynor@...el.com>
Subject: RE: [PATCH v14 5/5] x86/sgx: Enable automatic SVN updates for SGX
 enclaves



> -----Original Message-----
> From: Huang, Kai <kai.huang@...el.com>
> Sent: Friday, August 15, 2025 12:41 AM
> To: Reshetova, Elena <elena.reshetova@...el.com>; Hansen, Dave
> <dave.hansen@...el.com>
> Cc: seanjc@...gle.com; mingo@...nel.org; Scarlata, Vincent R
> <vincent.r.scarlata@...el.com>; x86@...nel.org; jarkko@...nel.org;
> Annapurve, Vishal <vannapurve@...gle.com>; linux-kernel@...r.kernel.org;
> Mallick, Asit K <asit.k.mallick@...el.com>; Aktas, Erdem
> <erdemaktas@...gle.com>; Cai, Chong <chongc@...gle.com>; Bondarevska,
> Nataliia <bondarn@...gle.com>; linux-sgx@...r.kernel.org; Raynor, Scott
> <scott.raynor@...el.com>
> Subject: Re: [PATCH v14 5/5] x86/sgx: Enable automatic SVN updates for SGX
> enclaves
> 
> On Thu, 2025-08-14 at 09:50 -0700, Dave Hansen wrote:
> > On 8/14/25 00:34, Elena Reshetova wrote:
> > > +/* Mutex to ensure no concurrent EPC accesses during EUPDATESVN */
> > > +static DEFINE_MUTEX(sgx_svn_lock);
> > > +
> > >  int sgx_inc_usage_count(void)
> > >  {
> > > +	int ret;
> > > +
> > > +	guard(mutex)(&sgx_svn_lock);
> > > +
> > > +	if (!sgx_usage_count) {
> > > +		ret = sgx_update_svn();
> > > +		if (ret)
> > > +			return ret;
> > > +	}
> > > +
> > > +	sgx_usage_count++;
> > > +
> > >  	return 0;
> > >  }
> > >
> > >  void sgx_dec_usage_count(void)
> > >  {
> > > -	return;
> > > +	sgx_usage_count--;
> > >  }
> >
> > How is a plain int-- safe?
> >
> > Where's the locking?
> 
> Sorry I missed this during review too.

My line of thinking went that we don't actually
care or act on decrement (it is not a true ref counter)
and therefore, races here are ok. What I forgot is that
we loose basic atomicity also with plain int ((

I would personally like to go back to atomic (this is
what it is exactly for), but I can also just add another
mutex here. Preferences? 

Best Regards,
Elena.