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Thu, 17 Dec 2020 20:55:44 +0000 (UTC) IronPort-SDR: dOIsc+csdAL0Ks8oGi5BV0OQxohxY6MEdJlGnI+vh7O6syOjZLw97mVzub4CcuwthTENTAm+Et KPCUdWThCIOA== X-IronPort-AV: E=McAfee;i="6000,8403,9838"; a="163065568" X-IronPort-AV: E=Sophos;i="5.78,428,1599548400"; d="scan'208";a="163065568" Received: from orsmga005.jf.intel.com ([10.7.209.41]) by orsmga106.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 17 Dec 2020 12:55:41 -0800 IronPort-SDR: Er/7M4PNlBvCC3Ot5KLxAnZGFDgXq3HrU66LwxKHt2s/DgLMMci6edfjYjpJASU1hKuz+dJcWz oxOi7UzuxYJQ== X-IronPort-AV: E=Sophos;i="5.78,428,1599548400"; d="scan'208";a="558188169" Received: from jlcowper-mobl1.amr.corp.intel.com (HELO [10.212.113.48]) ([10.212.113.48]) by orsmga005-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 17 Dec 2020 12:55:40 -0800 Subject: Re: [PATCH V3 10/10] x86/pks: Add PKS test code To: ira.weiny@intel.com, Thomas Gleixner , Ingo Molnar , Borislav Petkov , Andy Lutomirski , Peter Zijlstra , Dave Hansen Cc: Fenghua Yu , x86@kernel.org, linux-kernel@vger.kernel.org, Andrew Morton , linux-doc@vger.kernel.org, linux-nvdimm@lists.01.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, Dan Williams , Greg KH References: <20201106232908.364581-1-ira.weiny@intel.com> <20201106232908.364581-11-ira.weiny@intel.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20201106232908.364581-11-ira.weiny@intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 11/6/20 3:29 PM, ira.weiny@intel.com wrote: > + /* Arm for context switch test */ > + write(fd, "1", 1); > + > + /* Context switch out... */ > + sleep(4); > + > + /* Check msr restored */ > + write(fd, "2", 1); These are always tricky. What you ideally want here is: 1. Switch away from this task to a non-PKS task, or 2. Switch from this task to a PKS-using task, but one which has a different PKS value then, switch back to this task and make sure PKS maintained its value. *But*, there's no absolute guarantee that another task will run. It would not be totally unreasonable to have the kernel just sit in a loop without context switching here if no other tasks can run. The only way you *know* there is a context switch is by having two tasks bound to the same logical CPU and make sure they run one after another. This just gets itself into a state where it *CAN* context switch and prays that one will happen. You can also run a bunch of these in parallel bound to a single CPU. That would also give you higher levels of assurance that *some* context switch happens at sleep(). One critical thing with these tests is to sabotage the kernel and then run them and make *sure* they fail. Basically, if you screw up, do they actually work to catch it?