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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 2C0DCC433DF for ; Tue, 23 Jun 2020 09:45:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 16CFF20781 for ; Tue, 23 Jun 2020 09:45:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732095AbgFWJpY (ORCPT ); Tue, 23 Jun 2020 05:45:24 -0400 Received: from mx2.suse.de ([195.135.220.15]:45664 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731921AbgFWJpY (ORCPT ); Tue, 23 Jun 2020 05:45:24 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 07763AE8C; Tue, 23 Jun 2020 09:45:22 +0000 (UTC) Date: Tue, 23 Jun 2020 11:45:19 +0200 From: Joerg Roedel To: Andy Lutomirski Cc: Joerg Roedel , Dave Hansen , Tom Lendacky , Mike Stunes , Dan Williams , Dave Hansen , "H. Peter Anvin" , Juergen Gross , Jiri Slaby , Kees Cook , kvm list , LKML , Peter Zijlstra , 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: <20200623094519.GF31822@suse.de> References: <20200425191032.GK21900@8bytes.org> <910AE5B4-4522-4133-99F7-64850181FBF9@amacapital.net> <20200425202316.GL21900@8bytes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org Hi Andy, On Mon, Apr 27, 2020 at 10:37:41AM -0700, Andy Lutomirski wrote: > 1. Use IST for #VC and deal with all the mess that entails. With the removal of IST shifting I wonder what you would suggest on how to best implement an NMI-safe IST handler with nesting support. My current plan is to implement an IST handler which switches itself off the IST stack as soon as possible, freeing it for re-use. The flow would be roughly like this upon entering the handler; build_pt_regs(); RSP = pt_regs->sp; if (RSP in VC_IST_stack) error("unallowed nesting") if (RSP in current_kernel_stack) RSP = round_down_to_8(RSP) else RSP = current_top_of_stack() // non-ist kernel stack copy_pt_regs(pt_regs, RSP); switch_stack_to(RSP); To make this NMI safe, the NMI handler needs some logic too. Upon entering NMI, it needs to check the return RSP, and if it is in the #VC IST stack, it must do the above flow by itself and update the return RSP and RIP. It needs to take into account the case when PT_REGS is not fully populated on the return side. Alternativly the NMI handler could safe/restore the contents of the #VC IST stack or just switch to a special #VC-in-NMI IST stack. All in all it could get complicated, and imho shift_ist would have been simpler, but who am I anyway... Or maybe you have a better idea how to implement this, so I'd like to hear your opinion first before I spend too many days implementing something. Regards, Joerg