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 144A9C433EF for ; Mon, 11 Jul 2022 14:57:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230078AbiGKO5H (ORCPT ); Mon, 11 Jul 2022 10:57:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51590 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231866AbiGKO5E (ORCPT ); Mon, 11 Jul 2022 10:57:04 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 6233F71BC1 for ; Mon, 11 Jul 2022 07:57:03 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1657551422; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=RjKS13/NlMqaLdRkWRLl34SCYGjbP2YQ4ufi2FT/Xdk=; b=iRDOg0Vw7xqFj+Bg6tWfovVjLyJEFM9q2xk3+fjAcWO1OPpqSllL6Ok/TP25v7igpVZHTQ hHzOMQRQf8FaXj191bZLCO/HI8fX4ysfHSmcef6HY9qjKHA65v6I3lLXLFzT59eNZB8X8y 8bmPn62QyR8k1tnihbIdbjq7jjQMJbA= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-6-szgrPC89OX-dLPadLwjCKQ-1; Mon, 11 Jul 2022 10:56:55 -0400 X-MC-Unique: szgrPC89OX-dLPadLwjCKQ-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 0A818801590; Mon, 11 Jul 2022 14:56:55 +0000 (UTC) Received: from localhost (unknown [10.39.192.23]) by smtp.corp.redhat.com (Postfix) with ESMTPS id BA84A40D296C; Mon, 11 Jul 2022 14:56:54 +0000 (UTC) From: Cornelia Huck To: "Dr. David Alan Gilbert" , Peter Maydell Cc: Thomas Huth , Laurent Vivier , Eric Auger , Juan Quintela , qemu-arm@nongnu.org, qemu-devel@nongnu.org, kvm@vger.kernel.org Subject: Re: [PATCH RFC v2 0/2] arm: enable MTE for QEMU + kvm In-Reply-To: Organization: Red Hat GmbH References: <20220707161656.41664-1-cohuck@redhat.com> User-Agent: Notmuch/0.36 (https://notmuchmail.org) Date: Mon, 11 Jul 2022 16:56:53 +0200 Message-ID: <87r12r66kq.fsf@redhat.com> MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.84 on 10.11.54.2 Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Mon, Jul 11 2022, "Dr. David Alan Gilbert" wrote: > * Peter Maydell (peter.maydell@linaro.org) wrote: >> On Mon, 11 Jul 2022 at 14:24, Dr. David Alan Gilbert >> wrote: >> > But, ignoring postcopy for a minute, with KVM how do different types of >> > backing memory work - e.g. if I back a region of guest memory with >> > /dev/shm/something or a hugepage equivalent, where does the MTE memory >> > come from, and how do you set it? >> >> Generally in an MTE system anything that's "plain old RAM" is expected >> to support tags. (The architecture manual calls this "conventional >> memory". This isn't quite the same as "anything that looks RAM-like", >> e.g. the graphics card framebuffer doesn't have to support tags!) > > I guess things like non-volatile disks mapped as DAX are fun edge cases. > >> One plausible implementation is that the firmware and memory controller >> are in cahoots and arrange that the appropriate fraction of the DRAM is >> reserved for holding tags (and inaccessible as normal RAM even by the OS); >> but where the tags are stored is entirely impdef and an implementation >> could choose to put the tags in their own entirely separate storage if >> it liked. The only way to access the tag storage is via the instructions >> for getting and setting tags. > > Hmm OK; In postcopy, at the moment, the call qemu uses is a call that > atomically places a page of data in memory and then tells the vCPUs to > continue. I guess a variant that took an extra blob of MTE data would > do. Yes, the current idea is to extend UFFDIO_COPY with a flag so that we get the tag data along with the page. > Note that other VMMs built on kvm work in different ways; the other > common way is to write into the backing file (i.e. the /dev/shm > whatever atomically somehow) and then do the userfault call to tell the > vcpus to continue. It looks like this is the way things will work in > the split hugepage mechanism Google are currently adding. Hmm... I had the impression that other VMMs had not cared about this particular use case yet; if they need a slightly different mechanism, it would complicate things a bit.