From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.zytor.com (terminus.zytor.com [198.137.202.136]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 650641F03EF; Thu, 12 Jun 2025 18:12:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.137.202.136 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749751971; cv=none; b=Au7FxnU6iIMKSkPoAyaQM6aNC2+ZK+JzCc2rdqFPcs8Ihn/WjEja4UuqkcYvI1L60+Wmn1vsTgDBSoYF3AY7ftCiQcgkAxkc8pHVCZhXPKkHM1kZmoT6eck4BNHuDeJC16bsOmbk2kMQDWUNCaWh5CcJ9ftUc+/RxpKFxlW7bcY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749751971; c=relaxed/simple; bh=W6hcqUdXy8CqRMhltXJvswv0y/Bk2o7f+pYcaBXuBZg=; h=Message-ID:Date:MIME-Version:From:Subject:To:Cc:References: In-Reply-To:Content-Type; b=pcK4TqLpKdjQSFgHe9QEN0qBK2MFnK2yDeyoMZhWxm3+Wm6u0A+QtsyGrvQRdS/3F4onC+W4StA3cdHfOvXgSNqPgo0CD/Jyp9PU967qU/OG3rMUY64m0lCEkjPSNQU3E9c6OqMS0AaMI5ofhyECVix43vJf05C4/fa3BVtTCXQ= ARC-Authentication-Results:i=1; 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h=Date:From:Subject:To:Cc:References:In-Reply-To:From; b=MtqaZQgwwSTYESgLIeefZRce7wvVmLCvsZx6xbjkmOE3wz7AGuz5J9f4YVsqXI2IK lA9jOB5aLDfpg3ZbOtI5VR9sB92FfKAEXzM7o3qGwnKpGoIv68DqP27tryPqE5x/3X xhC0BoI0jQIBBvd226m5UNdOb3fa+snGh2OOzPXoYHbLI7tvHCC0RulAebwSiWZuiM vuv6YMiAsderQ4zJoOiW1HXdC9UgKH5chFYCt+LIX6tpsJWMhqH2xLT1rdNUQqFT6Z NKFj2brOg6vphGmawPgV5oOs4GT9wc4p55MyNudNEfn997Vthx6mWXnHERZ4sn1ScP dWBbSxMaxQ9kA== Message-ID: Date: Thu, 12 Jun 2025 11:12:11 -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 From: Xin Li Subject: Re: [PATCH v4 0/2] x86/fred: Prevent single-step upon ERETU completion To: Sohil Mehta , linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, shuah@kernel.org, andrew.cooper3@citrix.com References: <20250605181020.590459-1-xin@zytor.com> <8264e653-6570-4c2a-a9a7-c4104da26f76@intel.com> <06e68373-a92b-472e-8fd9-ba548119770c@intel.com> Content-Language: en-US Autocrypt: addr=xin@zytor.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 6/5/2025 2:51 PM, Sohil Mehta wrote: >>> Xin Li (Intel) (2): >>> x86/fred/signal: Prevent single-step upon ERETU completion >>> selftests/x86: Add a test to detect infinite sigtrap handler loop >>> >> I tested the patches on a machine that supports FRED. The results are as >> expected: >> > Seeing a split lock warning when running the test: > x86/split lock detection: #DB: sigtrap_loop_64/4614 took a bus_lock trap > at address: 0x4011ae > > Wanted to get this out sooner for awareness. Will figure out more > details and send out an update. We investigated this issue, and figured out the reason why we see a split lock warning when running the test: 1) The warning is not caused by the test. 2) It's a false warning. 3) It happens even when bus lock detection (BLD) is not enabled. 4) It happens only on the first #DB on a CPU. The root cause is that Linux writes 0 to DR6 at boot time, which results in different values in DR6 depending on whether they support BLD or not. On CPUs that support BLD, writing 0 to DR6 sets DR6 to 0xFFFF07F0, i.e., bit 11 of DR6, its BLD flag, is cleared. Otherwise 0xFFFF0FF0. But Intel SDM says, other than BLD induced #DB, DR6.BLD is not modified. To avoid confusion in identifying debug exceptions, software debug- exception handlers should set bit 11 to 1 before returning. DR6.BLD is always 1 if the processor does not support OS bus-lock detection. Because Linux clears DR6.BLD on CPUs that support BLD at boot time, the first #DB will be INCORRECTLY interpreted as a BLD #DB, thus the warning, no matter whether BLD is enabled or not. We will be working on a fix to initialize DR6 and DR7 with their architectural reset values instead of 0s. Thanks! Xin