From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-16.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DFEF5C433ED for ; Mon, 10 May 2021 08:08:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AE64D613C9 for ; Mon, 10 May 2021 08:08:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230193AbhEJIJ2 (ORCPT ); Mon, 10 May 2021 04:09:28 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:35742 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230093AbhEJIJ0 (ORCPT ); Mon, 10 May 2021 04:09:26 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1620634101; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=cbgsgg0fZcU1I+mnQQzyZz5vF48xBdx7km7JFxmZ8AY=; b=Uexn+TPFBvkdwaAl9biBbV7KVryz8l067iYvG9GTJBEQoEdHk5A3ng2tr/SHY4+a19bEWJ rMMH6UmvWdUzDyWyMxWAncEvEs76b4NPSjj1thr72BcOqQMEbiRuL3P7zWqMRxGmxme9DQ s/+8qB5zl16XBc6SWKq80kAWmJkF0+s= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-90-BLUM4czXPL2sRLahOcWQwA-1; Mon, 10 May 2021 04:08:18 -0400 X-MC-Unique: BLUM4czXPL2sRLahOcWQwA-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A974A6D24A; Mon, 10 May 2021 08:08:16 +0000 (UTC) Received: from starship (unknown [10.40.194.86]) by smtp.corp.redhat.com (Postfix) with ESMTP id C5E8D62680; Mon, 10 May 2021 08:08:13 +0000 (UTC) Message-ID: Subject: Re: [PATCH 02/15] KVM: x86: Emulate RDPID only if RDTSCP is supported From: Maxim Levitsky To: Sean Christopherson , Paolo Bonzini Cc: Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Xiaoyao Li , Reiji Watanabe Date: Mon, 10 May 2021 11:08:12 +0300 In-Reply-To: <20210504171734.1434054-3-seanjc@google.com> References: <20210504171734.1434054-1-seanjc@google.com> <20210504171734.1434054-3-seanjc@google.com> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.36.5 (3.36.5-2.fc32) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2021-05-04 at 10:17 -0700, Sean Christopherson wrote: > Do not advertise emulation support for RDPID if RDTSCP is unsupported. > RDPID emulation subtly relies on MSR_TSC_AUX to exist in hardware, as > both vmx_get_msr() and svm_get_msr() will return an error if the MSR is > unsupported, i.e. ctxt->ops->get_msr() will fail and the emulator will > inject a #UD. > > Note, RDPID emulation also relies on RDTSCP being enabled in the guest, > but this is a KVM bug and will eventually be fixed. > > Fixes: fb6d4d340e05 ("KVM: x86: emulate RDPID") > Cc: stable@vger.kernel.org > Signed-off-by: Sean Christopherson > --- > arch/x86/kvm/cpuid.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > index f765bf7a529c..c96f79c9fff2 100644 > --- a/arch/x86/kvm/cpuid.c > +++ b/arch/x86/kvm/cpuid.c > @@ -637,7 +637,8 @@ static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func) > case 7: > entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX; > entry->eax = 0; > - entry->ecx = F(RDPID); > + if (kvm_cpu_cap_has(X86_FEATURE_RDTSCP)) > + entry->ecx = F(RDPID); > ++array->nent; > default: > break; Just to make sure that I understand this correctly: This is what I know: Both RDTSCP and RDPID are instructions that read IA32_TSC_AUX (and RDTSCP also reads the TSC). Both instructions have their own CPUID bits (X86_FEATURE_RDPID, X86_FEATURE_RDTSCP) If either of these CPUID bits are present, IA32_TSC_AUX should be supported. RDPID is a newer feature, thus I can at least for the sanity sake assume that usually a CPU will either have neither of the features, have only RDTSCP, and IA32_AUX, or have both RDSCP and RDPID. If not supported in hardware KVM only emulates RDPID as I see. Why btw? Performance wise guest that only wants the IA32_AUX in userspace, is better to use RDTSCP and pay the penalty of saving/restoring of the unwanted registers, than use RDPID with a vmexit. My own guess for an answer to this question is that RDPID emulation is there to aid migration from a host that does support RDPID to a host that doesn't. Having said all that, assuming that we don't want to emulate the RDTSCP too, when it is not supported, then this patch does make sense. Reviewed-by: Maxim Levitsky Best regards, Maxim Levitsky