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 334D2E7C4E7 for ; Wed, 4 Oct 2023 16:55:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243551AbjJDQzC (ORCPT ); Wed, 4 Oct 2023 12:55:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50254 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243538AbjJDQzA (ORCPT ); Wed, 4 Oct 2023 12:55:00 -0400 Received: from mail-yw1-x1149.google.com (mail-yw1-x1149.google.com [IPv6:2607:f8b0:4864:20::1149]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 86FE8AD for ; Wed, 4 Oct 2023 09:54:57 -0700 (PDT) Received: by mail-yw1-x1149.google.com with SMTP id 00721157ae682-59f7d109926so33353577b3.2 for ; Wed, 04 Oct 2023 09:54:57 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1696438497; x=1697043297; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=v1SpCSVBVkGWfR+UVpLKMyMdKYaLIo/4ycal5KL0JmU=; b=BQCb8ij3zdhky/1vgEEKMhtBMsgfFAo6jps8xVr3wBICqOF/kjqhXaqdElAK2xI1bS U3cgOrlMvtFG5ECDU0wbJX7io9/m+60G84oEVcyPXvW5l+c99C+nkhMcjqMG06lAs64J JcsUd5KjOhbY3t9S2KlFc2Lqnf54zyOA4NMnRme9qoOxHc56ihSYzPgiCfXS1KTOHRLQ 7vqXSS9ZrMAqfViGVSYkaeO1rOdjaapK3UjLBKajtx9Ig7L9WIYQPNjuJczk7iHBYs5+ 7Bb3NbuRQ1K4k+rGFvupZ64bfmUnyG8EH0+/Tr4Rp2z2ywod6gN4uoG6+3Y42syQheqN al7g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1696438497; x=1697043297; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=v1SpCSVBVkGWfR+UVpLKMyMdKYaLIo/4ycal5KL0JmU=; b=bTsQPLfVHuHTmDiqG0Cjd6H+rKo8p9fgmGLU6pGR9DLdQBtLZp9MgiZD6kBRKKRhWs SR80qs2eU5sMy0wQXCW3iIzmafsJ2WXCtOxQpEIoLRCAyj59DUV0z+4aAvKAludI4ohQ nKqzkK42InH+IppQPZpEE6N7F4GG+sACqwW/TiyRWU9+o0EoqF9T6YR2OkilhyhHPa0Q /YGlE0b2IarKIa5YcfRanx/cBNkoNM6AlzD6kWx/9MdyMBulW6QOvoF4MIGbI7yyE16w lNRF1/flenLWxPN/2W8Us5qP0ALXBPCQszBdCTUzeRSyil4k8x+sSr6mc92WdNL4FyCK vwww== X-Gm-Message-State: AOJu0Yy5qq4vR6u08nOEKKAWaJbkFwUO2K8JeaQBaMPiRNdgU9N1wlaB fxZLMiiJK/a/3mSrtHSm9ZtYjSGEdXo= X-Google-Smtp-Source: AGHT+IF4i+oQKJd8u1V+TB1VPIOAogrhPtMWZdEvSZy8rU+jM1S65yQ94jKQ9NyvjRiXgJ/MLji9dcyTwZ4= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a81:a909:0:b0:59b:e97e:f7e3 with SMTP id g9-20020a81a909000000b0059be97ef7e3mr53564ywh.2.1696438496733; Wed, 04 Oct 2023 09:54:56 -0700 (PDT) Date: Wed, 4 Oct 2023 09:54:55 -0700 In-Reply-To: Mime-Version: 1.0 References: <20230928001956.924301-1-seanjc@google.com> Message-ID: Subject: Re: [PATCH 0/5] KVM: x86: Fix breakage in KVM_SET_XSAVE's ABI From: Sean Christopherson To: Tyler Stachecki Cc: Leonardo Bras , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, Paolo Bonzini , Shuah Khan , Nathan Chancellor , Nick Desaulniers , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org, llvm@lists.linux.dev Content-Type: text/plain; charset="us-ascii" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 04, 2023, Tyler Stachecki wrote: > On Wed, Oct 04, 2023 at 07:51:17AM -0700, Sean Christopherson wrote: > > It's not about removing features. The change you're asking for is to have KVM > > *silently* drop data. Aside from the fact that such a change would break KVM's > > ABI, silently ignoring data that userspace has explicitly requested be loaded for > > a vCPU is incredibly dangerous. > > Sorry if it came off that way No need to apologise, you got bit by a nasty kernel bug and are trying to find a solution. There's nothing wrong with that. > I fully understand and am resigned to the "you > break it, you keep both halves" nature of what I had initially proposed and > that it is not a generally tractable solution. Yeah, the crux of the matter is that we have no control or even knowledge of who all is using KVM, with what userspace VMM, on what hardware, etc. E.g. if this bug were affecting our fleet and for some reason we couldn't address the problem in userspace, carrying a hack in KVM in our internal kernel would probably be a viable option because we can do a proper risk assessment. E.g. we know and control exactly what userspace we're running, the underlying hardware in affected pools, what features are exposed to the guest, etc. And we could revert the hack once all affected VMs had been sanitized.