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X-CSE-ConnectionGUID: yaFN3KHsRPKHrOUn+sNRyw== X-CSE-MsgGUID: KM7h2N47S5avnTv6d71z1A== X-IronPort-AV: E=McAfee;i="6800,10657,11735"; a="92682811" X-IronPort-AV: E=Sophos;i="6.23,130,1770624000"; d="scan'208";a="92682811" Received: from orviesa003.jf.intel.com ([10.64.159.143]) by fmvoesa102.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 20 Mar 2026 08:50:55 -0700 X-CSE-ConnectionGUID: yG9x3Z6GQMm2jT4NTHpncQ== X-CSE-MsgGUID: WfzynnKxSVy2VH0H5oBc3Q== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.23,130,1770624000"; d="scan'208";a="227443022" Received: from dnelso2-mobl.amr.corp.intel.com (HELO [10.125.109.109]) ([10.125.109.109]) by ORVIESA003-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 20 Mar 2026 08:50:54 -0700 Message-ID: <568aa6c4-6802-4eb5-b412-e3aa93ed9b29@intel.com> Date: Fri, 20 Mar 2026 08:50:54 -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] selftests/x86: Fix sysret_rip assertion failure on FRED systems To: Andrew Cooper , yi1.lai@intel.com Cc: bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, luto@kernel.org, mingo@redhat.com, shuah@kernel.org, tglx@kernel.org, x86@kernel.org References: <43f2c5c2-6327-4546-b204-5c66b704042e@citrix.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/20/26 08:47, Andrew Cooper wrote: >> First, CPUID doesn't tell you if FRED is in use. Is it even on by >> default yet? There might not be a better way to do this than checking >> CPUID, but checking CPUID is imprecise at best. > A reliable way to distinguish IDT and FRED mode is to: > > 1) Load $3 into %fs (x86_64) or %gs (i386) (i.e. whichever isn't thread > local stoage) > 2) execute a breakpoint, ignore the signal > 3) Look to see whether %fs/%gs holds 3 or 0 > > IRET has a fun behaviour where it zeroes NULL selectors even if they had > a non-zero RPL. > > ERETU doesn't do this; Andy Luto and I asked for this minor information > leak to be removed, and Intel agreed as it served no purpose anyone > could identify. > > As a consequence, you can use it to determine whether the kernel used > IRET or ERET to return back to userspace. I was thinking of just grepping /proc/cpuinfo for "fred", but that sounds much more fun! :)