From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kai Huang Subject: Re: [PATCH] KVM: VMX: Fix commit which broke PML Date: Wed, 4 Nov 2015 13:43:07 +0800 Message-ID: <56399AEB.5050507@linux.intel.com> References: <1446529798-31895-1-git-send-email-kai.huang@linux.intel.com> <56388584.3080501@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit To: Paolo Bonzini , guangrong.xiao@linux.intel.com, kvm@vger.kernel.org Return-path: Received: from mga09.intel.com ([134.134.136.24]:10205 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751973AbbKDFrF (ORCPT ); Wed, 4 Nov 2015 00:47:05 -0500 In-Reply-To: <56388584.3080501@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 11/03/2015 05:59 PM, Paolo Bonzini wrote: > > On 03/11/2015 06:49, Kai Huang wrote: >> I found PML was broken since below commit: >> >> commit feda805fe7c4ed9cf78158e73b1218752e3b4314 >> Author: Xiao Guangrong >> Date: Wed Sep 9 14:05:55 2015 +0800 >> >> KVM: VMX: unify SECONDARY_VM_EXEC_CONTROL update >> >> Unify the update in vmx_cpuid_update() >> >> Signed-off-by: Xiao Guangrong >> [Rewrite to use vmcs_set_secondary_exec_control. - Paolo] >> Signed-off-by: Paolo Bonzini >> >> The reason is PML after above commit vmx_cpuid_update calls >> vmx_secondary_exec_control, in which PML is disabled unconditionally, as PML is >> enabled in creating vcpu. Therefore if vcpu_cpuid_update is called after vcpu is >> created, PML will be disabled unexpectedly while log-dirty code still think PML >> is used. Actually looks calling vmx_secondary_exec_control in vmx_cpuid_update >> is likely to break any VMX features that is enabled/disabled on demand by >> updating SECONDARY_VM_EXEC_CONTROL, if vmx_cpuid_update is called between the >> feature is enabled and disabled. >> >> Fix this by calling vmcs_read32 to read out SECONDARY_VM_EXEC_CONTROL directly. > vmx_cpuid_update() is meant to be mostly idempotent; the parts that > depend on the current VMCS configuration are hidden in > vmcs_set_secondary_control. So a better fix would be to add > SECONDARY_EXEC_ENABLE_PML to vmcs_set_secondary_exec_control's > "mask" variable. However, you can see from the comment: > > /* > * These bits in the secondary execution controls field > * are dynamic, the others are mostly based on the hypervisor > * architecture and the guest's CPUID. Do not touch the > * dynamic bits. > */ > > that even this is not the optimal fix. SECONDARY_EXEC_ENABLE_PML is > either always set or always clear, so it shouldn't be in "mask". > > Instead, it should be in vmcs_config.cpu_based_2nd_exec_ctrl. It isn't > because my review didn't notice this remnant of your original > implementation, which dynamically enabled/disabled PML. > > In fact, cpu_has_vmx_pml() expects SECONDARY_EXEC_ENABLE_PML to be set > in vmcs_config.cpu_based_2nd_exec_ctrl, so it is a bit confusing to > remove the bit unconditionally in vmx_secondary_exec_control! > > So I think SECONDARY_EXEC_ENABLE_PML should not be removed unconditionally > from exec_control in vmx_secondary_exec_control; the removal should be > conditional on !enable_pml, like we do for e.g. EPT or VPID. If you do this, > vmx_enable_pml and vmx_disable_pml need not touch SECONDARY_VM_EXEC_CONTROL > anymore. Do you agree? If so, can you prepare a patch along these lines? Thanks Paolo for your comments. Sure I agree. I will send out the v2 patch by following what you suggested. > > (Since you are at it, perhaps you can rename vmx_enable_pml and > vmx_disable_pml, since they will only allocate and free the PML page). I intend to rename vmx_enable{disable}_pml to vmx_create{destroy}_pml_buffer, as besides allocating buffer, we also need to write buffer address and PML index to VMCS. Thanks, -Kai > > Thanks for reporting the issue! > > Paolo > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >