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[213.175.37.10]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-702423c7c6dsm5116694b3a.1.2024.06.03.02.30.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 03 Jun 2024 02:30:28 -0700 (PDT) Date: Mon, 3 Jun 2024 11:30:22 +0200 From: Igor Mammedov To: Zhao Liu Cc: "Chen, Zide" , qemu-devel@nongnu.org, pbonzini@redhat.com, mst@redhat.com, thuth@redhat.com, cfontana@suse.de, xiaoyao.li@intel.com, qemu-trivial@nongnu.org Subject: Re: [PATCH V2 2/3] target/i386: call cpu_exec_realizefn before x86_cpu_filter_features Message-ID: <20240603113022.6569043b@imammedo.users.ipa.redhat.com> In-Reply-To: References: <20240524200017.150339-1-zide.chen@intel.com> <20240524200017.150339-3-zide.chen@intel.com> <04d3dfd8-93d2-493d-82d1-8fbcad6ecd22@intel.com> X-Mailer: Claws Mail 4.2.0 (GTK 3.24.41; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=imammedo@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -22 X-Spam_score: -2.3 X-Spam_bar: -- X-Spam_report: (-2.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.142, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Sat, 1 Jun 2024 23:26:55 +0800 Zhao Liu wrote: > On Fri, May 31, 2024 at 10:13:47AM -0700, Chen, Zide wrote: > > Date: Fri, 31 May 2024 10:13:47 -0700 > > From: "Chen, Zide" > > Subject: Re: [PATCH V2 2/3] target/i386: call cpu_exec_realizefn before > > x86_cpu_filter_features > > > > On 5/30/2024 11:30 PM, Zhao Liu wrote: > > > Hi Zide, > > > > > > On Fri, May 24, 2024 at 01:00:16PM -0700, Zide Chen wrote: > > >> Date: Fri, 24 May 2024 13:00:16 -0700 > > >> From: Zide Chen > > >> Subject: [PATCH V2 2/3] target/i386: call cpu_exec_realizefn before > > >> x86_cpu_filter_features > > >> X-Mailer: git-send-email 2.34.1 > > >> > > >> cpu_exec_realizefn which calls the accel-specific realizefn may expand > > >> features. e.g., some accel-specific options may require extra features > > >> to be enabled, and it's appropriate to expand these features in accel- > > >> specific realizefn. > > >> > > >> One such example is the cpu-pm option, which may add CPUID_EXT_MONITOR. > > >> > > >> Thus, call cpu_exec_realizefn before x86_cpu_filter_features to ensure > > >> that it won't expose features not supported by the host. > > >> > > >> Fixes: 662175b91ff2 ("i386: reorder call to cpu_exec_realizefn") > > >> Suggested-by: Xiaoyao Li > > >> Signed-off-by: Zide Chen > > >> --- > > >> target/i386/cpu.c | 24 ++++++++++++------------ > > >> target/i386/kvm/kvm-cpu.c | 1 - > > >> 2 files changed, 12 insertions(+), 13 deletions(-) > > >> > > >> diff --git a/target/i386/cpu.c b/target/i386/cpu.c > > >> index bc2dceb647fa..a1c1c785bd2f 100644 > > >> --- a/target/i386/cpu.c > > >> +++ b/target/i386/cpu.c > > >> @@ -7604,6 +7604,18 @@ static void x86_cpu_realizefn(DeviceState *dev, Error **errp) > > >> } > > >> } > > >> > > >> + /* > > >> + * note: the call to the framework needs to happen after feature expansion, > > >> + * but before the checks/modifications to ucode_rev, mwait, phys_bits. > > >> + * These may be set by the accel-specific code, > > >> + * and the results are subsequently checked / assumed in this function. > > >> + */ > > >> + cpu_exec_realizefn(cs, &local_err); > > >> + if (local_err != NULL) { > > >> + error_propagate(errp, local_err); > > >> + return; > > >> + } > > >> + > > >> x86_cpu_filter_features(cpu, cpu->check_cpuid || cpu->enforce_cpuid); > > > > > > For your case, which sets cpu-pm=on via overcommit, then > > > x86_cpu_filter_features() will complain that mwait is not supported. > > > > > > Such warning is not necessary, because the purpose of overcommit (from > > > code) is only to support mwait when possible, not to commit to support > > > mwait in Guest. > > > > > > Additionally, I understand x86_cpu_filter_features() is primarily > > > intended to filter features configured by the user, > > > > Yes, that's why this patches intends to let x86_cpu_filter_features() > > filter out the MWAIT bit which is set from the overcommit option. > > HMM, but in fact x86_cpu_filter_features() has already checked the MWAIT > bit set by "-overcommit cpu-pm=on". ;-) > > (Pls correct me if I'm wrong) Revisiting what cpu-pm did to MWAIT: > * Firstly, it set MWAIT bit in x86_cpu_expand_features(): > x86_cpu_expand_features() > -> x86_cpu_get_supported_feature_word() > -> kvm_arch_get_supported_cpuid() > This MWAIT is based on Host's MWAIT capability. This MWAIT enablement > is fine for next x86_cpu_filter_features() and x86_cpu_filter_features() > is working correctly here! > > * Then, MWAIT was secondly set in host_cpu_enable_cpu_pm() regardless > neither Host's support or previous MWAIT enablement result. This is > the root cause of your issue. > > Therefore, we should make cpu-pm honor his first MWAIT enablement result > instead of repeatly and unconditionally setting the MWAIT bit again in > host_cpu_enable_cpu_pm(). > > Additionally, I think the code in x86_cpu_realizefn(): > cpu->mwait.ecx |= CPUID_MWAIT_EMX | CPUID_MWAIT_IBE; > has the similar issue because it also should check MWAIT feature bit. > > Further, it may be possible to remove cpu->mwait: just check the MWAIT > bit in leaf 5 of cpu_x86_cpuid(), and if MWAIT is present, use host's > mwait info plus CPUID_MWAIT_EMX | CPUID_MWAIT_IBE. Agreed with above analysis, we shouldn't have host_cpu_enable_cpu_pm() as kvm_arch_get_supported_cpuid gets us MWAIT already. filling in cpu->mwait.ecx is benign mistake which likely doesn't trigger anything if CPUID_EXT_MONITOR is not present. But for clarity it's better to add an explicit check there as well. > > > > and the changes of > > > CPUID after x86_cpu_filter_features() should by default be regarded like > > > "QEMU knows what it is doing". > > > > Sure, we can add feature bits after x86_cpu_filter_features(), but I > > think moving cpu_exec_realizefn() before x86_cpu_filter_features() is > > more generic, and actually this is what QEMU did before commit 662175b91ff2. > > > > - Less redundant code. Specifically, no need to call > > x86_cpu_get_supported_feature_word() again. > > - Potentially there could be other features could be added from the > > accel-specific realizefn, kvm_cpu_realizefn() for example. And these > > features need to be checked against the host availability. > > Mainly I don't think this reorder is a direct fix for the problem (I > just analyse it above), also in your case x86_cpu_filter_features() will > print a WARNING when QEMU boots, which I don't think is cpu-pm's intention. There is no problem with warning, I'd even say it's a good thing. But you are right reordering just masks the issue. As for expected behavior, if user asked for "-overcommit cpu-pm=on" there are 2 options: * it's working as expected (mwait exiting is enabled successfully with CPUID MONITOR bit set) * QEMU shall fail to start.