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 A428BC54EE9 for ; Tue, 27 Sep 2022 17:08:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232253AbiI0RII (ORCPT ); Tue, 27 Sep 2022 13:08:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40184 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231236AbiI0RHv (ORCPT ); Tue, 27 Sep 2022 13:07:51 -0400 Received: from mail-pl1-x629.google.com (mail-pl1-x629.google.com [IPv6:2607:f8b0:4864:20::629]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 153AB87F9D for ; Tue, 27 Sep 2022 10:07:45 -0700 (PDT) Received: by mail-pl1-x629.google.com with SMTP id x1so9650215plv.5 for ; Tue, 27 Sep 2022 10:07:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date; bh=f0hIq2KYifdNKFEFb7aEPEmJFmBWVDO/Zv/NjHfHkzg=; b=kp2y1pD2TNHkVmFbKg/OSxw8KFbGNkM5ZdloYEPiV5zUAUrwEerxBdZKOJi0qRkwcM fp/Zhiqf547RsKsnk784wVwp3youq2kUThdSRzsrx7HU+53VR+sIgN3UatNFz/xtzjZX VA0U4dC2UnSJij6sWt4a4DCfqh3LsF7xhSHtZnlR+oUkOwNu7fsZzUbiaFE7UlXmmmW8 eSGzbAnmfMCZw9SAtwbi43UiPe7oTueIPEevMmHBOyKSblSqZltt/7rBOz4Xu0CfjeSW pUWt2/k8jfcvjjj9CbAWQDPsRZUjW59edFsi0exMCRXEgvqaFHQYwIrxElO4Gc2V4Qee vsUg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date; bh=f0hIq2KYifdNKFEFb7aEPEmJFmBWVDO/Zv/NjHfHkzg=; b=TZReGEm5qXabCcGncC6RfIiNnnl7eCoRpwN9uP4M2n9JkeGdUFag0zJy8jAaisl2Ln WAJMwu4uGSUm3ASUBoC2O06hYLBPWergEHB/1nujUpblsuvW2sRy+Kq/cgZl2l/+U73y A+//IkFS0MWd6p6p+Twhtiu/1kbfG3nCrL1KGIA/gFYg+1Mqr6uxUrB7MSqe7H3vR27y 3R9UMlyTLfycV/VGDAt2Ey6lX8hJS+S/4nECO+0LfvRNYqzH6CC3RiQ5Y/cjfZKbcOOd 4SROdYYGMJDiEgKm9te1/zJ3XAI+Rofh+alvAX72K5/bK6BwGndvRNr8AhIo1r0FrQIU RQYA== X-Gm-Message-State: ACrzQf2jpUIptriRjhqWboFhkvhMxsDJ/BPED7GxwhjlIZ6Ks3zvI7tj THg7RDC6YJiE4rogjC4+ieFiMA== X-Google-Smtp-Source: AMsMyM6d4d9UEcUbarBTqOcimdO4fhX+S9ZIQ+PNycGAaBaQZTVuUQHdnhU/14HYz93HVs9WNBULfw== X-Received: by 2002:a17:902:d512:b0:178:2898:8099 with SMTP id b18-20020a170902d51200b0017828988099mr27694224plg.131.1664298464296; Tue, 27 Sep 2022 10:07:44 -0700 (PDT) Received: from google.com (7.104.168.34.bc.googleusercontent.com. [34.168.104.7]) by smtp.gmail.com with ESMTPSA id y68-20020a623247000000b0053e7293be0bsm1991287pfy.121.2022.09.27.10.07.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 27 Sep 2022 10:07:43 -0700 (PDT) Date: Tue, 27 Sep 2022 17:07:39 +0000 From: Sean Christopherson To: Paolo Bonzini Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH] KVM: x86: disable on 32-bit unless CONFIG_BROKEN Message-ID: References: <20220926165112.603078-1-pbonzini@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220926165112.603078-1-pbonzini@redhat.com> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Mon, Sep 26, 2022, Paolo Bonzini wrote: > 32-bit KVM has extra complications in the code due to: > > - different ways to write 64-bit values in VMCS > > - different handling of DS and ES selectors as well as FS/GS bases > > - lack of CR8 and EFER > > - lack of XFD > More for the list: - SVM is effectively restricted to PAE kernels due to NX requirements > - impossibility of writing 64-bit PTEs atomically It's not impossible, just ugly. KVM could use CMPXCHG8B to do all of the accesses for the TDP MMU, including the non-atomic reads and writes. > The last is the big one, because it prevents from using the TDP MMU > unconditionally. As above, if the TDP MMU really is the sticking point, that's solvable. The real justification for deprecating 32-bit KVM is that, outside of KVM developers, literally no one uses 32-bit KVM. I.e. any amount of effort that is required to continue supporting 32-bit kernels is a complete waste of resources.