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" <zide.c...@intel.com> > 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 <zide.c...@intel.com> > >> 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 <xiaoyao...@intel.com> > >> Signed-off-by: Zide Chen <zide.c...@intel.com> > >> --- > >> 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. > > 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.