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 38AF4204C2F for ; Wed, 8 Jan 2025 23:33:18 +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=1736379199; cv=none; b=QcgDgZi2G34kMPsrdQOx4tzApRAxwbzVUC9UKS+raj6QqhGBsH12RktYLhp9vfQvx7yGZYshqeryZWK88/iV39umVDThuYB6J+J5RcxubOSNTJ1wKar8BnJCdRVWlhp5ai6Asg+xM4K11J+1C+2Vsy2kF62g/yYR4FC3/l9yvqY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1736379199; c=relaxed/simple; bh=aLcNYrAIwrtdl3He3i0zzTfpCSwH2HkkYpt/V4XAIlo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=nqP10rUmIsB/G2ZeUYiFP8eG1MCAxwrEK0MTrkP73EdTKF6LWmoJEld9PbHM2UNENrmiNHpVEcowVK4gxk668CndHxRUrja206ifNveYkUgc6zJja1PA4MDKBXV0wptHgQmPLGYIOndCsFmMBWFrgr2V6e0UdrojZOj4TDmj7Lk= ARC-Authentication-Results:i=1; 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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Lnd6HXEtONdk7PbCSJJO18B6RAxNTD9zByU+aipDxHaO0i9+soQuIsbLb4qu7dOel QVy0p905hoRkr4nGCb5MEtCN5EEvEJbhdtEu5pQ+uZvoa70wILT2A2YYNDKHDHMcUm NwQUD8W3sVV9RDCde4CeKP2s0eYJHtNh3xRwBXZjweKr0PvImx5cSOFPAVZLgBpb5e xDWw5dTrGqsr/izxrrK+ldeszo+pl7n+VwQEAMDduAiI6SIMo/rEFPvGo5Ivpotdz3 dftO7BF3GqEgG4OZOAzP02iJ+NvlAPCVMLGXl8WS2Nb4a9OTgtCNNigFmxve8z89U6 SDrAEAQEgIW+Q== Message-ID: <8f488a03-876b-481a-a20d-5d372fbe1dcf@zytor.com> Date: Wed, 8 Jan 2025 15:32:36 -0800 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 v1 1/1] x86/fred: Fix the FRED RSP0 MSR out of sync with its per CPU cache To: Dave Hansen , linux-kernel@vger.kernel.org Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, andrew.cooper3@citrix.com References: <20250108023641.17396-1-xin@zytor.com> Content-Language: en-US From: Xin Li Autocrypt: addr=xin@zytor.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 1/8/2025 3:04 PM, Dave Hansen wrote: > On 1/7/25 18:36, Xin Li (Intel) wrote: >> The FRED RSP0 MSR (pointing to the top of the kernel stack for user >> level event delivery) and its per CPU cache should be kept in sync to >> avoid redundant writes in the exit to user space path, as a result, >> a write to the FRED RSP0 MSR is paired with a write to its per CPU >> cache as fred_update_rsp0() does. > > I _think_ you're trying to explain the general use of a per-cpu MSR > cache. That's good. But I was reading this paragraph and thinking at > this point that the bug had something to do with redundant writes to the > MSR. > > How about this? > > The FRED RSP0 MSR is only used for delivering events when > running userspace. This kernel leverages this property to reduce > expensive MSR writes and optimize context switches. The kernel > only writes the MSR when about to run userspace *and* when the > MSR has actually changed since the last time userspace ran. > > This optimization is implemented by maintaining a per-cpu cache > of FRED RSP0 and then checking that against the value for the > current task's stack before running userspace. > > However cpu_init_fred_exceptions() writes the MSR without > updating the per-cpu cache. This means that the kernel might > return to userspace with MSR_IA32_FRED_RSP0==0 when it needed to > point to the current task stack. This would induce a double > fault (#DF), which is bad. > > A context switch after cpu_init_fred_exceptions() can paper over > the issue since it updates the cached value. That evidently > happens most of the time explaining how this bug got through. > This is a way better explanation, thanks a lot!