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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8C732C32772 for ; Wed, 17 Aug 2022 08:45:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232440AbiHQIph (ORCPT ); Wed, 17 Aug 2022 04:45:37 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41538 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238283AbiHQIpX (ORCPT ); Wed, 17 Aug 2022 04:45:23 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1C4BF74DC0 for ; Wed, 17 Aug 2022 01:45:21 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1660725921; 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=p/gDo76UwTmViEx13ZIEnXWDsVcvV6S3aUwfWuW0178=; b=ZDrsgY75BHyYvhC8kIYHQiEXAM/pqDF4mTxiqUqRLkqrdzp8+oIUU+XV6Y4CykYCNggnaS wcIS4+I2a3uNA/E+m2+Yz9oKnF5VUx4vv362J0G2g5zIPokdJIdB37lON5fjvCMPs/rZUE Xg5QLCOytwIjgkkN0Ou4u4DbUwDqySE= Received: from mail-ej1-f69.google.com (mail-ej1-f69.google.com [209.85.218.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_128_GCM_SHA256) id us-mta-615-R3kbIPctObOdu1vMgoBeUg-1; Wed, 17 Aug 2022 04:45:19 -0400 X-MC-Unique: R3kbIPctObOdu1vMgoBeUg-1 Received: by mail-ej1-f69.google.com with SMTP id sc31-20020a1709078a1f00b0073096c2b4e1so2727972ejc.22 for ; Wed, 17 Aug 2022 01:45:19 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:in-reply-to:subject:from:references:cc:to :content-language:user-agent:mime-version:date:message-id :x-gm-message-state:from:to:cc; bh=p/gDo76UwTmViEx13ZIEnXWDsVcvV6S3aUwfWuW0178=; b=4RLt8igO/3+ZtxLuW0FcJj+hKe3ITnEyf0EMGSfy6ZJBOqpGFp6R5RRe9FNpJMwsFA hPMw28zv5T34p5jYkw3zE6+HF3uz+vJVTef8BiNPmlFs88oaFwyG+1DOcqS8JZlxjcYo RH1ykH47WO+B5ZBqORAkrRusTvBjJRjlGY+fA3N+k+ofryFa7OXa9/eThEegCzXEyrxM zH6eW6A0Odg/U3VGIPZsOgZorA0rowog7lnJy2AfUVPwB/1RFofoyElQ2af6u60Wa7jE tV8UxZBfc/+T+WBZciROueX5kP9+r9wl8b/dm2kjk3qTsNOQQhiG/ac9x027S6AIDJyQ 2Ygw== X-Gm-Message-State: ACgBeo384ijYP6Zm7zuiupiO8Txc1zs3gbkHnqWJCh2yr1lVkCZRsQSh leIKKGLSbbDvUD0wsMNuhruvydq7c/+Hcff5Ix9y7mgrwuccMJfqoQFmieoBzotSoQYrlfbyMTl rLLjdehcXFL7x3aWVWqI9Y5nw X-Received: by 2002:a05:6402:35c:b0:43c:8f51:130 with SMTP id r28-20020a056402035c00b0043c8f510130mr22069657edw.393.1660725918837; Wed, 17 Aug 2022 01:45:18 -0700 (PDT) X-Google-Smtp-Source: AA6agR5AMm80dBw8wUUUR+UqHcBujLtUvJk4USLHFXwkNf0tAYNyaMXZlXOZmkAx/1CzOJRTLVTo1A== X-Received: by 2002:a05:6402:35c:b0:43c:8f51:130 with SMTP id r28-20020a056402035c00b0043c8f510130mr22069645edw.393.1660725918646; Wed, 17 Aug 2022 01:45:18 -0700 (PDT) Received: from ?IPV6:2001:b07:6468:f312:9af8:e5f5:7516:fa89? ([2001:b07:6468:f312:9af8:e5f5:7516:fa89]) by smtp.googlemail.com with ESMTPSA id o2-20020a170906358200b00730979f568fsm6393950ejb.150.2022.08.17.01.45.16 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 17 Aug 2022 01:45:18 -0700 (PDT) Message-ID: Date: Wed, 17 Aug 2022 10:45:14 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Content-Language: en-US To: =?UTF-8?Q?Leonardo_Br=c3=a1s?= , Sean Christopherson , "Dr. David Alan Gilbert (git)" Cc: kvm@vger.kernel.org, tglx@linutronix.de, linux-kernel@vger.kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org References: <20220816175936.23238-1-dgilbert@redhat.com> From: Paolo Bonzini Subject: Re: [PATCH] KVM: x86: Always enable legacy fp/sse In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/17/22 05:29, Leonardo Brás wrote: >>> QEMU always calls kvm_put_xsave, even on this combination because >>> KVM_CAP_CHECK_EXTENSION_VM always returns true for KVM_CAP_XSAVE. > Any particular reason why it always returns true for KVM_CAP_XSAVE, even when > the CPU does not support it? > > IIUC, if it returns false to this capability, kvm_put_xsave() should never be > called, and thus it can avoid bug reproduction. Because it allows userspace to have a single path for saving/restoring FPU state. See for example the "migration" code in tools/testing/selftests/kvm/lib/x86_64/processor.c (the vcpu_save_state and vcpu_load_state functions). In fact, the QEMU code that uses KVM_GET_FPU/KVM_SET_FPU in x86 is obsolete, because it's not been used since Linux 2.6.36. Paolo