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([2a02:6b6a:b566:0:8009:2525:9580:8db2]) by smtp.gmail.com with ESMTPSA id q14-20020a05600c46ce00b003db12112fcfsm3212547wmo.4.2023.02.09.12.32.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 09 Feb 2023 12:32:03 -0800 (PST) Message-ID: <9b6bca9c-7189-a2d5-8c0a-f55c24f54b62@bytedance.com> Date: Thu, 9 Feb 2023 20:32:02 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.2 Subject: Re: [External] Re: [PATCH v8 8/9] x86/mtrr: Avoid repeated save of MTRRs on boot-time CPU bringup Content-Language: en-US To: Thomas Gleixner , dwmw2@infradead.org, kim.phillips@amd.com Cc: arjan@linux.intel.com, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, x86@kernel.org, pbonzini@redhat.com, paulmck@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, rcu@vger.kernel.org, mimoja@mimoja.de, hewenliang4@huawei.com, thomas.lendacky@amd.com, seanjc@google.com, pmenzel@molgen.mpg.de, fam.zheng@bytedance.com, punit.agrawal@bytedance.com, simon.evans@bytedance.com, liangma@liangbit.com, David Woodhouse References: <20230209154156.266385-1-usama.arif@bytedance.com> <20230209154156.266385-9-usama.arif@bytedance.com> <87mt5m1yiz.ffs@tglx> From: Usama Arif In-Reply-To: <87mt5m1yiz.ffs@tglx> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/02/2023 18:31, Thomas Gleixner wrote: > On Thu, Feb 09 2023 at 15:41, Usama Arif wrote: >> void mtrr_save_state(void) >> { >> + static bool mtrr_saved; >> int first_cpu; >> >> if (!mtrr_enabled()) >> return; >> >> + if (system_state < SYSTEM_RUNNING) { >> + if (!mtrr_saved) { >> + mtrr_save_fixed_ranges(NULL); >> + mtrr_saved = true; >> + } >> + return; >> + } >> + >> first_cpu = cpumask_first(cpu_online_mask); >> smp_call_function_single(first_cpu, mtrr_save_fixed_ranges, NULL, 1); > > So why is this relevant after the initial bringup? The BP MTRRs have > been saved already above, no? > > Thanks, > > tglx I will let David confirm if this is correct and why he did it, but this is what I thought while reviewing before posting v4: - At initial boot (system_state < SYSTEM_RUNNING), when mtrr_save_state is called in do_cpu_up at roughly the same time so MTRR is going to be the same, we can just save it once and then reuse for other secondary cores as it wouldn't have changed for the rest of the do_cpu_up calls. - When the system is running and you offline and then online a CPU, you want to make sure that hotplugged CPU gets the current MTRR (which might have changed since boot?), incase the MTRR has changed after the system has been booted, you save the MTRR of the first online CPU. When the hotplugged CPU runs its initialisation code, its fixed-range MTRRs will be updated with the newly saved fixed-range MTRRs. So mainly for hotplug, but will let David confirm. Thanks, Usama