From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754849AbcHBGu3 (ORCPT ); Tue, 2 Aug 2016 02:50:29 -0400 Received: from mail-wm0-f68.google.com ([74.125.82.68]:33607 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752254AbcHBGuU (ORCPT ); Tue, 2 Aug 2016 02:50:20 -0400 MIME-Version: 1.0 In-Reply-To: References: <1468351223-3250-1-git-send-email-pbonzini@redhat.com> <1468351223-3250-5-git-send-email-pbonzini@redhat.com> <20160713203006.GB16130@potion> <402de949-31fb-1733-9479-4803fce7de93@redhat.com> From: Wanpeng Li Date: Tue, 2 Aug 2016 09:05:40 +0800 Message-ID: Subject: Re: [RFC PATCH 4/4] KVM: vmx: add support for emulating UMIP To: Paolo Bonzini Cc: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , "linux-kernel@vger.kernel.org" , kvm Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2016-08-01 23:01 GMT+08:00 Paolo Bonzini : > > > On 31/07/2016 04:32, Wanpeng Li wrote: >> 2016-07-14 16:09 GMT+08:00 Paolo Bonzini : >> [...] >>> >>> This is not necessary because this is how KVM computes >>> CPUID[EAX=7,EBX=0].ECX: >>> >>> unsigned f_umip = kvm_x86_ops->umip_emulated() ? F(UMIP) : 0; >>> ... >>> const u32 kvm_cpuid_7_0_ecx_x86_features = F(PKU) | F(UMIP); >>> ... >>> // Mask userspace-provided value against supported features >>> entry->ecx &= kvm_cpuid_7_0_ecx_x86_features; >>> // Mask userspace-provided value against host features >>> cpuid_mask(&entry->ecx, CPUID_7_ECX); >>> // Finally add emulated features >>> entry->ecx |= f_umip; >> >> I think you mean: >> >> - entry->ecx -> userspace-provided value >> - kvm_cpuid_7_0_ecx_x86_features -> supported features >> - CPUID_7_ECX -> host features >> >> However, entry->ecx is returned by cpuid instruction >> (do_cpuid_1_ent()), so why it is a userspace-provided value? > > You're right, it's this: > > // Mask host processor value against supported features > entry->ecx &= kvm_cpuid_7_0_ecx_x86_features; > // Mask host processor value further, e.g. to drop > // features that the host kernel has blacklisted. > cpuid_mask(&entry->ecx, CPUID_7_ECX); > // Finally add emulated features > entry->ecx |= f_umip; > Cool, more clear this time. :) > The idea is the same. :) > > On the other hand, it is true that in many cases of the "switch > (function)" the call to do_cpuid_1_ent is unnecessary, and instead of > cpuid_mask you could just access boot_cpu_data.x86_capability[wordnum]. Agreed. Regards, Wanpeng Li