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X-CSE-ConnectionGUID: Ro5Hf7ZFRQKzctoUZtVgFw== X-CSE-MsgGUID: +EjAXw1bQiS3ph8xPMW9IQ== X-IronPort-AV: E=McAfee;i="6700,10204,11130"; a="28830606" X-IronPort-AV: E=Sophos;i="6.09,201,1716274800"; d="scan'208";a="28830606" Received: from orviesa006.jf.intel.com ([10.64.159.146]) by fmvoesa103.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 11 Jul 2024 15:30:38 -0700 X-CSE-ConnectionGUID: GfOzNDmMSoGQDY6CfG5tlg== X-CSE-MsgGUID: 3tsDK+TjTyW6sjrNIh/kuw== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.09,201,1716274800"; d="scan'208";a="49129533" Received: from unknown (HELO [10.124.221.144]) ([10.124.221.144]) by orviesa006-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 11 Jul 2024 15:30:38 -0700 Message-ID: <4bba0c20-0cd0-4c1a-abf0-511ba6940a57@intel.com> Date: Thu, 11 Jul 2024 15:30:35 -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 0/6] Introduce CET supervisor state support To: "Edgecombe, Rick P" , "Yang, Weijiang" , "seanjc@google.com" , "x86@kernel.org" , "linux-kernel@vger.kernel.org" , "tglx@linutronix.de" , "kvm@vger.kernel.org" , "pbonzini@redhat.com" Cc: "john.allen@amd.com" , "peterz@infradead.org" , "Gao, Chao" , "mlevitsk@redhat.com" References: <20240531090331.13713-1-weijiang.yang@intel.com> <67c5a358-0e40-4b2f-b679-33dd0dfe73fb@intel.com> <1c2fd06e-2e97-4724-80ab-8695aa4334e7@intel.com> <7df3637c85517f5bc4e3583249f919c1b809f370.camel@intel.com> Content-Language: en-US From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/11/24 15:11, Edgecombe, Rick P wrote: > On Thu, 2024-07-11 at 13:58 -0700, Dave Hansen wrote: >> So we're down to choosing between >> >> * $BYTES space in 'struct fpu' (on hardware supporting CET-S) >> >> or >> >> * ~100 loc >> >> $BYTES is 24, right? Did I get anything wrong? > > Do we know what the actual memory use is? It would increases the size asked of > of the allocator by 24 bytes, but what amount of memory actually gets reserved? > > It is sometimes a slab allocated buffer, and sometimes a vmalloc, right? I'm not > sure about slab sizes, but for vmalloc if the increase doesn't cross a page > size, it will be the same size allocation in reality. Or if it is close to a > page size already, it might use a whole extra 4096 bytes. Man, I hope I don't have this all mixed up in my head. Wouldn't be the first time. I _think_ you might be confusing thread_info and thread_struct, though. I know I've gotten them confused before. But we get to the 'struct fpu' via: current->thread.fpu Where current is a 'task_struct' which is in /proc/slabinfo and 'struct thread_struct thread' and 'struct fpu' are embedded in 'task_struct', not allocated on their own: task_struct 2958 3018 10048 3 8 ... So my current task_struct is 10048 bytes and 3 of them fit in each 8-page slab, leaving 2624 bytes to spare. I don't think we're too dainty about adding thing to task_struct. Are we? > So we might be looking at a situation where some tasks get an entire extra page > allocated per task, and some get no difference. And only the average is 24 bytes > increase. I think you're right here, at least when it comes to large weirdly-sized slabs. But _so_ many things affect task_struct that I've never seen anyone sweat it too much.