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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id CF1DBC30653 for ; Thu, 4 Jul 2024 09:40:02 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1sPIwE-0005rw-ME; Thu, 04 Jul 2024 05:39:38 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1sPIwC-0005kX-GF for qemu-devel@nongnu.org; Thu, 04 Jul 2024 05:39:36 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1sPIw6-00073O-A8 for qemu-devel@nongnu.org; Thu, 04 Jul 2024 05:39:35 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1720085969; h=from:from:reply-to: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=wDLZJHKzSrjg2/Q2Ck/6gMTQyX9tZqJNy+6mBsYfN4w=; b=VkFk2PllrNxPP62Q2DlRKaYxxRCm/UnQgkyzMpTXG4gzgPIRv65BVrJTay/r40h63Uxf62 0wwNm43SrE33vF9rwvX0j6pWMXUcdxI6qjAmaPulGgqcNJOAaPkkBE6kaOnn9BOexfDf31 mMIT3N+c4vgIUMaiBOricgcuZPtC1uQ= Received: from mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-583-OZfNEKOQO9ugSNal8hkWRQ-1; Thu, 04 Jul 2024 05:39:25 -0400 X-MC-Unique: OZfNEKOQO9ugSNal8hkWRQ-1 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id DC4201958B35; Thu, 4 Jul 2024 09:39:23 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.21]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id E92353000180; Thu, 4 Jul 2024 09:39:20 +0000 (UTC) Date: Thu, 4 Jul 2024 10:39:17 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Paolo Bonzini Cc: Michael Roth , qemu-devel@nongnu.org, kvm@vger.kernel.org Subject: Re: [PATCH] i386/sev: Don't allow automatic fallback to legacy KVM_SEV*_INIT Message-ID: References: <20240704000019.3928862-1-michael.roth@amd.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.12 (2023-09-09) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, T_SPF_TEMPERROR=0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Thu, Jul 04, 2024 at 11:31:16AM +0200, Paolo Bonzini wrote: > On Thu, Jul 4, 2024 at 10:42 AM Daniel P. Berrangé wrote: > > > > On Thu, Jul 04, 2024 at 08:51:05AM +0200, Paolo Bonzini wrote: > > > On Thu, Jul 4, 2024 at 2:01 AM Michael Roth wrote: > > > > Currently if the 'legacy-vm-type' property of the sev-guest object is > > > > left unset, QEMU will attempt to use the newer KVM_SEV_INIT2 kernel > > > > interface in conjunction with the newer KVM_X86_SEV_VM and > > > > KVM_X86_SEV_ES_VM KVM VM types. > > > > > > > > This can lead to measurement changes if, for instance, an SEV guest was > > > > created on a host that originally had an older kernel that didn't > > > > support KVM_SEV_INIT2, but is booted on the same host later on after the > > > > host kernel was upgraded. > > > > > > I think this is the right thing to do for SEV-ES. I agree that it's > > > bad to require a very new kernel (6.10 will be released only a month > > > before QEMU 9.1), on the other hand the KVM_SEV_ES_INIT API is broken > > > in several ways. As long as there is a way to go back to it, and it's > > > not changed by old machine types, not using it for SEV-ES is the > > > better choice for upstream. > > > > Broken how ? I know there was the regression with the 'debug_swap' > > parameter, but was something that should just be fixed in the kernel, > > rather than breaking userspace. What else is a problem ? > > The debug_swap parameter simply could not be enabled in the old API > without breaking measurements. The new API *is the fix* to allow using > it (though QEMU doesn't have the option plumbed in yet). There is no > extensibility. > > Enabling debug_swap by default is also a thorny problem; it cannot be > enabled by default because not all CPUs support it, and also we'd have > the same problem that we cannot enable debug_swap on new machine types > without requiring a new kernel. Tying the default to the -cpu model > would work but it is confusing. Presumably we can tie it to '-cpu host' without much problem, and then just leave it as an opt-in feature flag for named CPU models. > But I guess we can add support for debug_swap, disabled by default and > switch to the new API if debug_swap is enabled. > > > I don't think its reasonable for QEMU to require a brand new kernel > > for new machine types, given SEV & SEV-ES have been deployed for > > many years already. > > I think it's reasonable if the fix is displayed right into the error > message. It's only needed for SEV-ES though, SEV can use the old and > new APIs interchangeably. FYI currently it is proposed to unconditionally force set legacy-vm-type=true in libvirt, so QEMU guests would *never* use the new ioctl, to fix what we consider to be a QEMU / kernel guest ABI regression. If libvirt adds this though, it is basically a forever setting we would never be able to remove as removing would break ABI compat again. https://lists.libvirt.org/archives/list/devel@lists.libvirt.org/message/KP24LPFV4OCMN45TDHNXQVBPDZ56QSRR/ I'd much rather QEMU did NOT set this by default in its machine types, so libvirt doesn't have to add this forced flag. That way downstream distros who /can/ guarantee new enough kernels, can still use legacy-vm-type=false in their downstream machine types, without libvirt overriding this. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|