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X-CSE-ConnectionGUID: 0WpNUkmYQ3esFwrmIzqdPg== X-CSE-MsgGUID: usrBL9TBQICg0Y3o8Km67A== X-IronPort-AV: E=McAfee;i="6800,10657,11491"; a="54831925" X-IronPort-AV: E=Sophos;i="6.16,311,1744095600"; d="scan'208";a="54831925" Received: from orviesa002.jf.intel.com ([10.64.159.142]) by orvoesa110.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 14 Jul 2025 08:37:53 -0700 X-CSE-ConnectionGUID: MbG7zrlKTOu5SP974B/DCQ== X-CSE-MsgGUID: ck7x8PG4Q2CG3rhGDHOP7A== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.16,311,1744095600"; d="scan'208";a="187947800" Received: from dwesterg-mobl1.amr.corp.intel.com (HELO [10.125.111.82]) ([10.125.111.82]) by orviesa002-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 14 Jul 2025 08:37:51 -0700 Message-ID: Date: Mon, 14 Jul 2025 08:37:50 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 5/5] x86/sgx: Enable automatic SVN updates for SGX enclaves From: Dave Hansen To: "Reshetova, Elena" Cc: "jarkko@kernel.org" , "seanjc@google.com" , "Huang, Kai" , "mingo@kernel.org" , "linux-sgx@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "x86@kernel.org" , "Mallick, Asit K" , "Scarlata, Vincent R" , "Cai, Chong" , "Aktas, Erdem" , "Annapurve, Vishal" , "dionnaglaze@google.com" , "Bondarevska, Nataliia" , "Raynor, Scott" References: <20250711165212.1354943-1-elena.reshetova@intel.com> <20250711165212.1354943-6-elena.reshetova@intel.com> <461a33d3-f91a-4dd3-bb97-048670530b25@intel.com> <2cd81800-12ba-460e-ac28-4532d0be1ec9@intel.com> Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/14/25 06:54, Dave Hansen wrote: > On 7/14/25 00:35, Reshetova, Elena wrote: >> I think you put it: this would require a spinlock in the fast path and >> *in theory* if we are running many many concurrent enclaves can create >> contention > FWIW, my mental model is that spinlocks that are held for short periods > of time are pretty much the same cost as an atomic under contention. > > If there are lots of users, the cost of moving the cacheline for the > atomic or the spinlock dominates everything else. It doesn't matter > whether that cacheline is an atomic_t or spinlock_t. > > The only difference is that there is _visible_ spinning for a spinlock. Oh, and I had a brain fart on this one. You've got a mutex, not a spinlock. But the concept still applies: for small critical sections, the cost of moving the cacheline dominates the cost of everything else, no matter if it's a mutex, spinlock or atomic. Never add complexity unless you're getting actual, real-world performance out of it. In this case, the only thing you'd _maybe_ improve with added complexity is an open()/close() loop on /dev/sgx, which is completely unrealistic.