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=-0.8 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 A6A48C433DF for ; Tue, 23 Jun 2020 12:52:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7F8172072E for ; Tue, 23 Jun 2020 12:52:35 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="maQA3A/n" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732542AbgFWMwb (ORCPT ); Tue, 23 Jun 2020 08:52:31 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39566 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729504AbgFWMwb (ORCPT ); Tue, 23 Jun 2020 08:52:31 -0400 Received: from casper.infradead.org (unknown [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2F3CAC061573; Tue, 23 Jun 2020 05:52:31 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=LyGXCQbBjA1RL7EQ24/1+IBfI7cHxTXkkAPcsBmfZf4=; b=maQA3A/n4fXnwedsyFDIlCzshd 3P/+FfPylnGfzbb/LR6DWp0V05k/lUCN+MHXRqb0xn1bSpQnEiUR4/ZLFReWYZoHAPugDh+gBnrrn Fam7mb61MDRG+GDY+s6D85u+S2yiGVQVuU0xht6f/6Dq2HmeIYGMaffZeXxcRT+OoeyKIHZkwb1yX gumYwQjenmsiuQhTvIa3ESIXCcRkMszYMcmm5rSRkvjblLzqFfUJezUFGPQkhiQKXmKLCpmbsrX9S XNkqILJIAAQIvhGB6fjzwcqinSZlgdIT2ON3w76tyiKcxteTqIKUjwErXAZAJ4o36Ygmelcn31Mi/ DVr4iJcw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1jniPE-0002Zz-Fj; Tue, 23 Jun 2020 12:52:04 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 13AEB300F28; Tue, 23 Jun 2020 14:52:02 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 04FE72370F7C3; Tue, 23 Jun 2020 14:52:02 +0200 (CEST) Date: Tue, 23 Jun 2020 14:52:01 +0200 From: Peter Zijlstra To: Joerg Roedel Cc: Andy Lutomirski , Joerg Roedel , Dave Hansen , Tom Lendacky , Mike Stunes , Dan Williams , Dave Hansen , "H. Peter Anvin" , Juergen Gross , Jiri Slaby , Kees Cook , kvm list , LKML , Thomas Hellstrom , Linux Virtualization , X86 ML , Sean Christopherson , Andrew Cooper Subject: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace) Message-ID: <20200623125201.GG4817@hirez.programming.kicks-ass.net> References: <20200425191032.GK21900@8bytes.org> <910AE5B4-4522-4133-99F7-64850181FBF9@amacapital.net> <20200425202316.GL21900@8bytes.org> <20200428075512.GP30814@suse.de> <20200623110706.GB4817@hirez.programming.kicks-ass.net> <20200623113007.GH31822@suse.de> <20200623114818.GD4817@hirez.programming.kicks-ass.net> <20200623120433.GB14101@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200623120433.GB14101@suse.de> Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Tue, Jun 23, 2020 at 02:04:33PM +0200, Joerg Roedel wrote: > On Tue, Jun 23, 2020 at 01:48:18PM +0200, Peter Zijlstra wrote: > > On Tue, Jun 23, 2020 at 01:30:07PM +0200, Joerg Roedel wrote: > > > But you cannot do a recursion check in #VC, because the NMI can happen > > on the first instruction of #VC, before we can increment our counter, > > and then the #VC can happen on NMI because the IST stack is a goner, and > > we're fscked again (or on a per-cpu variable we touch in our elaborate > > NMI setup, etc..). > > No, the recursion check is fine, because overwriting an already used IST > stack doesn't matter (as long as it can be detected) if we are going to > panic anyway. It doesn't matter because the kernel will not leave the > currently running handler anymore. You only have that guarantee when any SNP #VC from kernel is an automatic panic. But in that case, what's the point of having the recursion count? > > I'll keep repeating this, x86_64 exceptions are a trainwreck, and IST in > > specific is utter crap. > > I agree, but don't forget the most prominent underlying reason for IST: > The SYSCALL gap. If SYSCALL would switch stacks most of those issues > would not exist. IST would still be needed because there are no task > gates in x86-64, but still... We could all go back to int80 ;-) /me runs like heck